Yb: and nd: mode-locked oscillators and fiber systems incorporated in solid-state short pulse laser systems

ABSTRACT

The invention describes classes of robust fiber laser systems usable as pulse sources for Nd: or Yb: based regenerative amplifiers intended for industrial settings. The invention modifies adapts and incorporates several recent advances in FCPA systems to use as the input source for this new class of regenerative amplifier.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.12/340,036, filed Dec. 19, 2008, which is a divisional of U.S.application Ser. No. 11/005,218 filed Dec. 7, 2004, which is thedisclosure of which is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

Regenerative amplifiers utilizing chirped pulse amplification (CPA) havebeen the dominant means for obtaining pulse energies greater than amicrojoule with pulse durations in the femtosecond to picosecond range.Microjoule to millijoule pulse energies with pulse durations below 10picoseconds have been found to be particularly useful for micromachiningand for medical applications such as Lasik. However, a big stumblingblock in the utilization of ultrafast sources for these applications hasbeen that the regenerative amplifier is more of a piece of laboratoryequipment and not conducive to the industrial setting.

Alternative sources for microjoule level, ultrafast pulses are emerging;utilizing all fiber chirped pulse amplification designs. Such systemsare inherently more stable since they are based on technology similar tothat utilized in Telecomm systems. During the past decade, there hasbeen intensive work and success in making such systems practical.However, for higher pulse energies in the millijoule range, regenerativeamplifiers will continue to dominate for some time since pulse energiesabove a millijoule have not been demonstrated in an all fiber system.

For micromachining applications, more industrially compatibleregenerative amplifiers are now being developed based on Nd: or Yb:doped materials, rather than the Ti:sapphire that has dominated thescientific market. There are two basic reasons for this change.Commercial markets typically do not require the shorter pulses that canonly be obtained from the Ti:sapphire regenerative amplifier, and theNd: and Yb: based materials can be directly diode pumped, which makesthese systems more robust and less expensive. An unresolved technicalissue for Nd: or Yb: based regenerative amplifiers is the need for anequally robust seed source for femtosecond or picosecond pulses. Thepresent seed lasers are mode-locked solid-state lasers with questionablereliability. It would be preferable to have a robust fiber seed sourcesimilar to that which has been developed for the Ti:sapphireregenerative amplifier, and used where Ti:sapphire regenerativeamplifiers are applied to more commercial applications.

In a copending U.S. application Ser. No. 10/960,923, filed which isassigned to the common assignee and the disclosure of which isincorporated by reference in its entirety, the design changes needed fora mode-locked Yb:doped fiber oscillator and amplifier to be utilized asa seed source for a Yb: or Nd: based solid-state regenerative amplifierare described. The purpose of this application is to modify and applymany of the improvements in all fiber chirped pulse amplificationsystems for application to the seed source of a regenerative amplifier.

SUMMARY OF THE INVENTION

The purpose of this invention is to incorporate the many recentimprovements in femtosecond mode-locked fiber lasers and femtosecondfiber chirped pulse amplification systems to regenerative amplifiersystems that incorporate femtosecond or picosecond pulse sources basedon fiber seed-sources and/or fiber amplifiers.

Yb: and Nd: mode-locked oscillators with fiber amplifiers can beutilized as sources of ultrafast pulses for regenerative amplifiers inorder to obtain higher pulse energies than can be realized at this timefrom all fiber short pulse systems. A8827 (incorporated by referenceherein) describes specifically how the sources can be configured to beimplemented in such fiber based seed sources for solid stateregenerative amplifiers. The femtosecond source and fiber amplifier needto be carefully configured in order to obtain optimum, reliableperformance when incorporated into such a system. Recently there havebeen many improvements in mode-locked fiber sources implemented withfiber amplifiers in chirped pulse amplifier systems that can be utilizedin a regenerative amplifier system that typically is based on chirpedpulse amplification. Applicable improvements to fiber mode-lockedsources are disclosed in Ser. Nos. 09/576,772, 09/809,248, 10/627,069,10/814,502 and 10/814,319 (all incorporated by reference herein).Alternative suitable femtosecond sources that utilize fiberamplification for pulse conditioning and shortening are described inSer. No. 10/437,057. One of the difficulties with chirped pulseamplification systems has been in producing reliable and compact pulsestretchers that can be dispersion matched to pulse compressors suitablefor high pulse energies.

Significant improvements for dispersion matched fiber stretchers forfiber based chirped pulse amplification are disclosed in Attorney DocketNo. A8717, filed Nov. 22, 2004 (incorporated by reference herein). Theseimprovements are also applicable to chirped pulse amplification systemseven when solid state bulk mode-locked lasers are utilized as the seedsource. Significant improvements have been made in packaging, electroniccontrols, fabrication processes and optical parameter controls in orderto make fiber based femtosecond sources reliable. These engineeringimprovements can also be utilized in these regenerative amplifiersystems and are disclosed in Ser. Nos. 10/606,829, 10/813,163,10/813,173 and Attorney Docket No. A8828 (all incorporated by referenceherein).

Previously, Yb: and Nd: mode-locked oscillators and fiber amplifiershave been utilized as pulse sources for narrow bandwidth, bulk,solid-state amplifiers including regenerative amplifiers that canproduce pulses 20 picoseconds or greater. In general, the configurationsolutions for these longer pulse sources as described, for example inSer. No. 10/927,374 (incorporated by reference herein) are differentthan those described here for sub-picosecond systems. However, theengineering improvements described here will also be applicable for thelonger pulse systems, and the bulk amplifier operated as a regenerativeamplifier has increased flexibility.

The first important element for a short pulse regenerative amplifiersystem is the source of short pulses. Femtosecond mode-locked fiberlasers are a good source of such pulses. Typically the fiber oscillatoris low power and needs additional amplification for application as aseed source. Other important needs are pulse compression, wavelengthflexibility, dispersion control and fiber delivery.

Therefore, it is an object of the present invention to introduce amodular, compact, widely-tunable, high peak and high average power, lownoise ultrafast fiber amplification laser system suitable for a seedsource for a regenerative amplifier.

It is a further object of the invention to ensure modularity of thesystem by employing a variety of easily interchangeable optical systems,such as 1) short pulse seed sources, 2) wide bandwidth fiber amplifiers,3) dispersive pulse stretching elements, 4) dispersive pulse compressionelements, 5) nonlinear frequency conversion elements and 6) opticalcomponents for fiber delivery. In addition, any of the suggested modulescan be comprised of a subset of interchangeable optical systems.

It is a further object of the invention to ensure system compactness byemploying efficient fiber amplifiers, directly or indirectly pumped bydiode lasers as well as highly integrated dispersive delay lines. Thehigh peak power capability of the fiber amplifiers is greatly expandedby using parabolic or other optimized pulse shapes. In conjunction withself-phase modulation, parabolic pulses allow for the generation oflarge-bandwidth high-peak power pulses, as well as for well-controlleddispersive pulse stretching. High power parabolic pulses are generatedin high-gain single or multi-mode fiber amplifiers operating atwavelengths where the fiber material dispersion is positive.

Parabolic pulses can be delivered or transmitted along substantial fiberlengths even in the presence of self-phase modulation or generalKerr-effect type optical nonlinearities, while incurring only asubstantially linear pulse chirp. At the end of such fiber delivery orfiber transmission lines, the pulses can be compressed to approximatelytheir bandwidth limit.

Further, the high energy capability of fiber amplifiers is greatlyexpanded by using chirped pulse amplification in conjunction withparabolic pulses or other optimized pulse shapes, which allow thetoleration of large amounts of self-phase modulation without adegradation of pulse quality. Highly integrated chirped pulseamplification systems are constructed without compromising thehigh-energy capabilities of optical fibers by using fiber-based pulsestretchers in conjunction with bulk-optic pulse compressors (or lownonlinearity Bragg gratings) or periodically poled nonlinear crystals,which combine pulse compression with frequency-conversion.

The dispersion in the fiber pulse stretcher and bulk optic compressor ismatched to quartic order in phase by implementing fiber pulse stretcherswith adjustable 2nd, 3rd and 4th order dispersion. Adjustablehigher-order dispersion can be obtained by using high numerical aperturesingle-mode fibers with optimized refractive index profiles by itself orby using standard step-index high numerical aperture fibers inconjunction with linearly chirped fiber gratings. Alternatively,higher-order dispersion can be controlled by using the dispersiveproperties of the higher-order mode in a high numerical aperturefew-moded fiber, by using nonlinearly chirped fiber gratings or by usinglinearly chirped fiber gratings in conjunction with transmissive fibergratings. Adjustable 4th order dispersion can be obtained by controllingthe chirp in fiber Bragg gratings, transmissive fiber gratings and byusing fibers with different ratios of 2^(nd), 3^(rd) and 4^(th) orderdispersion. Equally, higher-order dispersion control can be obtained byusing periodically poled nonlinear crystals.

The fiber amplifiers are seeded by short pulse laser sources, preferablyin the form of short pulse fiber sources. For the case of Yb fiberamplifiers, Raman-shifted and frequency doubled short pulse Er fiberlaser sources can be implemented as widely tunable seed sources. Tominimize the noise of frequency conversion from the 1.5 μm to the 1.0 μmregime, self-limiting Raman-shifting of the Er fiber laser pulse sourcecan be used. Alternatively, the noise of the nonlinear frequencyconversion process can be minimized by implementing self-limitingfrequency-doubling, where the center wavelength of the tuning curve ofthe doubling crystal is shorter than the center wavelength of theRaman-shifted pulses.

The process of Raman-shifting and frequency-doubling can also beinverted, where an Er fiber laser is first frequency-doubled andsubsequently Raman-shifted in an optimized fiber providingsoliton-supporting dispersion for wavelengths around 800 nm and higherto produce a seed source for the 1 μm wavelength regime.

As an alternative low-complexity seed source for an Yb amplifier, amodelocked Yb fiber laser can be used. The fiber laser can be designedto produce strongly chirped pulses and an optical filter can beincorporated to select near bandwidth-limited seed pulses for the Ybamplifier.

Presently the mode-locked Yb: doped fiber laser is the preferredoscillator. The preferred source is described Ser. No. 10/627,069(incorporated herein).

The present invention is similarly directed to a mass-produciblepassively modelocked fiber laser. By incorporating apodized fiber Bragggratings, integrated fiber polarizers and concatenated sections ofpolarization-maintaining and non-polarization-maintaining fibers, afiber pig-tailed, linearly polarized output can be readily obtained fromthe laser. By further matching the dispersion value of the fiber Bragggrating to the inverse, or negative, of the dispersion of theintra-cavity fiber, the generation of optimally short pulses with alarge optical bandwidth can be induced. In this regard, either positivedispersion in conjunction with negative dispersion fiber gratings ornegative dispersion in conjunction with positive dispersion fibergratings can be implemented. Preferably, the dispersion characteristicsof the fiber Bragg grating and the dispersion characteristics of therest of the intra-cavity elements are matched to within a factor ofthree. Even more preferably, these characteristics are matched within afactor of two, or within a factor in the range of 1.0 to 2.0. Alsopreferably, the Bragg grating has a chirp rate greater than 80 nm/cm.More preferably, the Bragg grating has a chirp rate greater than 160nm/cm. Most preferably, the Bragg grating has a chirp rater greater than300 nm/cm. To maximize the output power and the pulse repetition rate,the use of wide-bandwidth fiber Bragg gratings with low absolutedispersion is preferable. These fiber Bragg gratings are also used asend-mirrors for the cavity and allow the transmission of pump light tothe intra-cavity gain fiber. The fiber Bragg gratings are convenientlyproduced using phase masks.

Alternatively, fiber couplers can be used inside the fiber cavity.Generally, sections of polarization-maintaining andnon-polarization-maintaining fiber can be concatenated inside the fibercavity. The non-polarization-maintaining section should then be shortenough so as not to excessively perturb the polarization state.Intra-cavity sections of non-polarization-maintaining fiber preferablycomprise all-fiber polarizers to lead to preferential oscillation of onelinear polarization state inside the cavity. Similarly, when directlyconcatenating polarization-maintaining fiber sections, the length of theindividual section should be long enough to prevent coherentinteractions of pulses propagating along the two polarization axes ofthe polarization-maintaining fibers, thereby ensuring a maximum in pulsestability.

Saturable absorber mirrors (SAMs) placed inside the cavity enablepassive modelocking. The saturable absorbers (SA) can be made frommultiple quantum wells (MQW) or bulk semiconductor films. Thesesaturable absorbers have preferably a bi-temporal life-time with a slowcomponent (>>100 ps) and a fast component (<<20 ps). The realization ofthe bi-temporal dynamics of the optical nonlinearity is achieved bytailoring the depth profile of the ion-implantation in combination withthe implantation dose and energy. The result is that the carriers trapat distinctively different rates in different depth regions of the SAM.

Saturating semiconductor films can for example be grown fromaluminum-containing material such as AlGaInAs, the exact composition canbe selected depending on the sought band-gap (typically selected to bein the vicinity of the desired operating wavelength of the laser system)and it is also governed by the requirement of lattice-match between thesaturating semiconductor film and an underlying Bragg mirror or anyother adjacent semiconductor material. Compositional requirementsenabling lattice match between semiconductors and/or a certain band gapare well known in the state of the art and are not further explainedhere.

In aluminum containing semiconductors the surface area can induce a lowoptical damage threshold triggered by oxidization of the surface. Inorder to prevent optical damage of aluminum containing surface areas apassivation layer, e.g., InP, InGaAs or GaAs, is incorporated. SAdegradation is further minimized by optimizing the optical beam diameterthat impinges on the SAM. In one implementation the SAM and anintra-cavity fiber end can be either butt-coupled or brought into closecontact to induce modelocking. Here, the incorporation of a precisionAR-coating on the intra-cavity fiber end minimizes any bandwidthrestrictions from etalon formation between the SAM and the fiber end.Etalons can also be minimized by appropriate wedging of the fiber ends.The beam diameter inside the SAM can be adjusted by implementing fiberends with thermally expanded cores. Alternatively, focusing lenses canbe directly fused to the fiber end. Moreover, graded-index lenses can beused for optimization of the focal size and working distance between thefiber tip and SA surface.

Wavelength tuning of the fiber lasers can be obtained by heating,compression or stretching of fiber Bragg gratings or by theincorporation of bulk optic tuning elements.

The use of bi- or multi-temporal saturable absorbers allows the designof dispersion compensated fiber laser operating in a single-polarizationstate, producing pulses at the bandwidth limit of the fiber gain medium.

Further improvement of the femtosecond Yb doped fiber oscillator caninclude an integral mass produced master oscillator, power amplifierdesign (MOPA) which is describe in Ser. No. 10/814,502 (incorporated byreference herein).

One embodiment of the present invention comprises a master oscillatorpower amplifier comprising a mode-locked fiber oscillator and a fiberamplifier. The mode-locked fiber oscillator comprises a pair ofreflective optical elements that form an optical resonator. At least oneof the reflective optical elements is partially transmissive and has areflection coefficient that is less than about 60%. The mode-lockedfiber oscillator outputs a plurality of optical pulses. The fiberamplifier is optically connected to the mode-locked fiber oscillatorthrough a bi-directional optical connection such that light from themode-locked fiber oscillator can propagate to the fiber amplifier andlight from the fiber amplifier can propagate to the mode-locked fiberoscillator.

Another embodiment of the present invention comprises a method ofproducing laser pulses. In this method, optical energy is propagatedback and forth through a gain fiber by reflecting light from a pair ofreflective elements on opposite ends of the gain fiber. Less than about60% of the light in the gain fiber is reflected back into the gain fiberby one of the reflectors. The pair of reflective elements together forma resonant cavity that supports a plurality of resonant optical modes.The resonant optical modes are substantially mode-locking to produce atrain of pulses. The train of optical pulses is propagated from thelaser cavity through one of the reflectors to a fiber amplifier along abi-directional optical path from the laser cavity to the fiber amplifierwhere the laser pulses are amplified.

Another embodiment of the present invention comprises a fiber-basedmaster oscillator power amplifier comprising a mode-locked fiberoscillator, a fiber amplifier comprising a gain fiber, andbi-directional optical path between the mode-locked fiber oscillator andthe fiber amplifier. The mode-locked fiber oscillator comprises aresonant cavity and a gain medium. The mode-locked fiber oscillatorproduces a plurality of optical pulses. The bi-directional optical pathbetween the mode-locked fiber oscillator and the fiber amplifier permitslight from the mode-locked fiber oscillator to propagate to the fiberamplifier and light from the fiber amplifier to propagate to themode-locked fiber oscillator. The mode-locked fiber oscillator comprisesa first segment of fiber and the fiber amplifier comprises a secondsegment of optical fiber. The first and second segments form asubstantially continuous length of optical fiber. In some embodiments,the first and second segments are spliced together. The first and secondsegments may be fusion spliced. The first and second segments may alsobe butt coupled together with or without a small gap, such as a smallair gap, between the first and second segments.

Another embodiment of the present invention comprises a method ofproducing laser pulses comprising substantially mode-lockinglongitudinal modes of a laser cavity to produce laser pulses andpropagating the laser pulses from the laser cavity to a fiber amplifier.The laser pulses are amplified in the fiber amplifier. Amplifiedspontaneous emission emitted from the fiber amplifier is received at thelaser cavity. A first portion of the spontaneous emission enters thelaser cavity. A second portion of the amplified spontaneous emissionfrom the laser cavity is retro-reflected back to the fiber amplifier tocause the second portion to be directed away from the cavity toward thefiber amplifier.

Another embodiment of the present invention comprises a fiber masteroscillator power amplifier comprising a mode-locked fiber oscillator anda fiber amplifier. The mode-locked fiber oscillator comprises a firstportion of optical fiber and a pair of reflectors spaced apart to form afiber optic resonator in the first fiber portion. At least one of thefiber reflectors comprises a partially transmissive fiber reflector. Themode-locked fiber oscillator outputs a plurality of optical pulses. Thefiber amplifier comprises a second portion of optical fiber opticallyconnected to the partially transmissive fiber reflector to receive theoptical pulses from the mode-locked oscillator. The second portion ofoptical fiber has gain to amplify the optical pulses. The first portionof optical fiber, the partially transmissive fiber reflector, and thesecond portion of optical fiber comprise a continuous path formed byoptical fiber uninterrupted by non-fiber optical components.

Another embodiment of the present invention comprises a masteroscillator power amplifier comprising a mode-locked fiber oscillator anda fiber amplifier. The mode-locked fiber oscillator comprises a pair ofreflective optical elements that form an optical resonator. At least oneof the reflective optical elements comprises a partially transmissiveBragg fiber grating having a reflection coefficient that is less thanabout 60%. The mode-locked fiber oscillator outputs a plurality ofoptical pulses. A fiber amplifier is optically connected to theoscillator through an optical connection to the partially transmissiveBragg fiber grating.

Another embodiment of the present invention comprises a masteroscillator power amplifier comprising a mode-locked fiber oscillator, afiber amplifier, and a pump source. The mode-locked fiber oscillatorcomprises a pair of reflective optical elements that form an opticalresonator. At least one of the reflective optical elements is partiallytransmissive and has a reflection coefficient that is less than about60%. The mode-locked fiber oscillator outputs a plurality of opticalpulses. A fiber amplifier is optically connected to the oscillatorthrough an optical connection to the at least one partially transmissivereflective optical elements. The pump source is optically connected tothe mode-locked fiber oscillator and the fiber amplifier to pump themode-locked fiber oscillator and the fiber amplifier.

However, for most embodiments for a source for a regenerative amplifierthe pulses need to be conditioned before amplification. Ser. No.10/814,319 (incorporated by reference herein) addresses the utilizationof modules so that the correct performance can be obtained from thefemtosecond source for the seeder or a portion of the seeder for theregenerative amplifier system. Parameter controls available throughthese modules can be utilized for the optimization of the output fromthe regenerative amplifier.

One embodiment of the invention thus comprises a pulsed fiber laseroutputting pulses having a duration and corresponding pulse width. Thepulsed laser comprises a modelocked fiber oscillator, an amplifier, avariable attenuator, and a compressor. The modelocked fiber oscillatoroutputs optical pulses. The amplifier is optically connected to themodelocked fiber oscillator to receive the optical pulses. The amplifiercomprises a gain medium that imparts gain to the optical pulse. Thevariable attenuator is disposed between the modelocked fiber oscillatorand the amplifier. The variable attenuator has an adjustabletransmission such that the optical energy that is coupled from themode-locked fiber oscillator to the amplifier can be reduced. Thecompressor compresses the pulse thereby reducing the width of the pulse.Preferably a minimum pulse width is obtained.

Another embodiment of the invention comprises a method of producingcompressed high power short laser pulses having an optical power of atleast about 200 mW and a pulse duration of about 200 femtoseconds orless. In this method, longitudinal modes of a laser cavity aresubstantially mode-locked to repetitively produce a laser pulse. Thelaser pulse is amplified. The laser pulse is also chirped therebychanging the optical frequency of the optical pulse over time. The laserpulse is also compressed by propagating different optical frequencycomponents of the laser pulse differently to produce compressed laserpulses having a shortened temporal duration. In addition, the laserpulse is selectively attenuated prior to the amplifying of the laserpulse to further shorten the duration of the compressed laser pulses.

Another embodiment of the invention comprises a method of manufacturinga high power short pulse fiber laser. This method comprises mode-lockinga fiber-based oscillator that outputs optical pulses. This methodfurther comprises optically coupling an amplifier to the fiber-basedoscillator through a variable attenuator so as to feed the opticalpulses from the fiber-based oscillator through the variable attenuatorand to the amplifier. The variable attenuator is adjusted based on ameasurement of the optical pulses to reduce the intensity of the opticalpulses delivered to the amplifier and to shorten the pulse.

Another embodiment of the invention comprises a pulsed fiber laseroutputting pulses having a pulse width. The pulsed fiber laser comprisesa modelocked fiber oscillator, an amplifier, and a spectral filter. Themodelocked fiber oscillator produces an optical output comprising aplurality of optical pulses having a pulse width and a spectral powerdistribution having a bandwidth. The amplifier is optically connected tothe modelocked fiber amplifier for amplifying the optical pulses. Thespectral filter is disposed to receive the optical output of themodelocked fiber oscillator prior to reaching the amplifier. Thespectral filter has a spectral transmission with a band edge thatoverlaps the spectral power distribution of the optical output of themodelocked fiber oscillator to attenuate a portion of the spectral powerdistribution and thereby reduce the spectral bandwidth. The pulse widthof the optical pulses coupled from the mode locked fiber oscillator tothe fiber amplifier is thereby reduced.

Another embodiment of the invention comprises a method of producingcompressed optical pulses. In this method, longitudinal modes of a fiberresonant cavity are substantially mode-locked so as to produce a trainof optical pulses having a corresponding spectral power distributionwith a spectral bandwidth. The optical pulses are amplified andcompressed to produce compressed optical pulses. The spectral bandwidthof the spectral power distribution is reduced such that the compressedoptical pulses have a shorter duration.

Another embodiment of the invention comprises a pulsed fiber lasercomprising a modelocked fiber oscillator, an amplifier, one or moreoptical pump sources, a pulse compressor, and a pre-compressor. Themodelocked fiber oscillator comprises a gain fiber and a pair ofreflective optical elements disposed with respect to the gain fiber toform a resonant cavity. The modelocked fiber oscillator produces a trainof optical pulses having an average pulse width. The amplifier isoptically connected to the modelocked fiber amplifier such that theoptical pulses can propagate through the amplifier. The fiber amplifieramplifies the optical pulses. The one or more optical pump sources areoptically connected to the modelocked fiber oscillator and the fiberamplifier to pump the fiber oscillator and fiber amplifier. The pulsecompressor is optically coupled to receive the amplified optical pulsesoutput from fiber amplifier. The pulse compressor shortens the pulsewidth of the optical pulses output by the fiber amplifier. Thepre-compressor is disposed in an optical path between the modelockedfiber oscillator and the fiber amplifier. The pre-compressor shortensthe duration of the optical pulses introduced into the fiber amplifiersuch that the pulse duration of the optical pulses output by thecompressor can be further shortened.

Another embodiment of the invention comprises a method of generatingshort high power optical pulses. The method comprises substantiallymode-locking optical modes of a laser cavity to produce an opticalsignal comprising a plurality of laser pulses having an average pulsewidth. The optical signal comprises a distribution of frequencycomponents. The method further comprises compressing the optical pulsesand amplifying the compressed optical pulses to produce amplifiedcompressed optical pulses. The amplified compressed optical pulses arefurther compressed subsequent to the amplifying using a dispersiveoptical element to differentiate between spectral components andintroducing different phase shifts to the different spectral components.

Another embodiment of the invention comprises a pulsed fiber lasercomprising a modelocked fiber oscillator, a fiber amplifier, an opticalpump source, and a pulse compressor. The modelocked fiber oscillatoroutputs optical pulses. The fiber amplifier is optically connected tothe modelocked fiber oscillator and amplifies the optical pulses. Theoptical pump source is optically connected to the fiber amplifier. Thepulse compressor is optically coupled to receive the amplified opticalpulses output from the fiber amplifier. The pulsed fiber laser furthercomprises at least one of (i) a first optical tap in the optical pathbetween the modelocked fiber oscillator and the fiber amplifier and afirst feedback loop from the first tap to control the modelocked fiberoscillator based on measurement of output from the first optical tap,and (ii) a second optical tap in the optical path between the fiberamplifier and the compressor and a second feedback loop from the secondtap to control the fiber amplifier based on measurement of output fromthe first optical tap.

Another embodiment of the invention comprises a pulsed light sourcecomprising a light source module, an isolator module, an amplifiermodule, and a compressor module. The light source module comprises anoptical fiber and outputs optical pulses. The isolator module comprisesan optical isolator in a housing having input and output fibers. Theinput fiber is optically coupled to the optical fiber of the lightsource module. The optical isolator is disposed in an optical pathconnecting the input and output fibers such that the optical pulsesintroduced into the input fiber are received by the isolator andpermitted to continue along the optical path to the output coupler. Theamplifier module comprises an amplifying medium and has an optical inputoptically connected to the output fiber of the isolator module toamplify the optical pulses. The compressor module is optically coupledto the amplifier module to compress the optical pulses.

Up to this point a mode-locked fiber laser or a bulk solid statemode-locked laser as the seed source for the fiber amplifier andregenerative amplifier has been disclosed. Other sources can also beutilized such a laser-diodes or microchip lasers. In Ser. No. 10/437,057(incorporated by reference herein), it is disclosed how to modify thesesources to give higher energy and shorter pulses through amplificationand pulse compression in fiber amplifiers. An advantage of these sourcesthat is mentioned in Ser. No. 10/437,057 is the repetition rate can bevariable. It is a true advantage to match the repetition rate of thesource to that of the regenerative amplifier.

Thus, one object of this invention is to convert relatively long pulsesfrom rep-rate variable ultrafast optical sources to shorter, high-energypulses suitable for seed sources in high-energy ultrafast lasersincluding a regenerative amplifier. Another object of this invention isto take advantage of the need for higher pulse energies at lowerrepetition rates so that such sources can be cost effective.

A gain switched laser diode as is used in telecom systems can be used asthe initial source of pulses. In this case, the diode is operated at amuch lower repetition rate. The pulses are still amplified in fiberamplifiers. Fiber amplifiers can be used as constant output powerdevices. The upper-state lifetime in typical doped amplifier fibers suchas Ytterbium and Erbium is in the millisecond range so that theseamplifiers can amplify pulse trains with the same efficiency atrepetition rates from 10's of kHz to 100's of GHz and beyond. If theamplifier is amplifying pulses at 10 kHz rather than at 10 GHz atconstant power, then the pulse energy will be six orders of magnitudehigher. Again, with such high peak powers, pulse compression methodsneed to be different and unique. One first embodiment uses conventionalcompression by spectral broadening the pulses in an optical fiber withpositive group velocity dispersion (GVD) and then compressing the pulsewith diffraction gratings. The object of the pulse compression is toconvert the 3-25 picosecond pulses from the gain switched laser diode topulses that are subpicosecond.

Another source starts with pulses from a low cost Q-switched microchiplaser. These lasers give pulses as short as 50 picoseconds but typically250 picoseconds to 1.0 nanosecond. The pulse peak powers are typically1-10 kW with pulse energies 6 orders of magnitude higher than fromtelecom laser diodes. Microchip lasers could be a very cost effectivesource for pulses less than 10 picoseconds with suitable pulsecompression methods. Single mode fiber compression has thus far beenlimited to pulses shorter than 150 ps and peak powers less than 1 kW.Before compression the pulse can be further amplified in a regenerativeamplifier.

Once a suitable femtosecond source has been identified furtherimprovements have been made in the incorporation of these lasers inchirped pulse amplification systems where the amplifier has been a fiberamplifier. In Ser. No. 10/813,163, many improvements to the fiberchirped pulse amplification (FCPA) configuration have been made for aconfiguration that is more robust and suitable to an industrialenvironment. Here it has been realized that these improvements can bealso utilized for fiber lasers seeding solid state amplifiers andparticular solid state regenerative amplifiers. Specifically, theimprovements for the FCPA configuration that are disclosed in Ser. No.10/813,163 can be utilized in a regenerative amplifier seeded with afiber laser configuration. The simplest embodiments are for thereplacement of the power amplifier in FIGS. 1 and 11 of this applicationwith a regenerative amplifier.

The following topics that are covered in Ser. No. 10/813,163 arerelevant to this configuration.

-   -   1) Functional segmentation of opto-mechanical components into        modular devices to produce manufacturable industrial laser        systems with Telcordia-grade quality and reliability.    -   2) Polarization fidelity within and between modules    -   3) Provision for tap units for test, monitoring or feedback    -   4) Spectral matching of oscillator to amplifier    -   5) Selection of the length of an amplifier to cut ASE at the        lasing wavelength    -   6) Active stabilization of the optical performance of gain fiber        in a laser or amplifier. The stabilization is realized by        actively adjusting the pump source wavelength by changing the        source temperature in order to match pump wavelength with the        absorption spectrum of the gain medium. The temperature        dependent spectrum in the gain fiber is cloned in the same type        of fiber, and thus used as a monitor. Accurate control of the        gain performance over a wide range of operating temperatures is        possible implementing this method.    -   7) Extraction of one or more chirped pulses from a series of        such pulses using an acousto-optic deflector, and compensation        for detrimental effects on the spatial characteristics of the        extracted chirped pulse, caused by dispersion in that deflector.

The invention thus relates to the technologies necessary to overcome theabove problems and limitations of the prior art, to build a hybrid fiberand solid-state based chirped pulse amplification laser system suitablefor industrial applications, with the fiber in a modular and compactlaser design with all modules replaceable. The modules are designed andmanufactured to telecom standards and quality.

Environmentally stable laser design is crucial for industrialapplication. An industrial laser system can be, for example,characterized by an output power variation below 0.5 dB over anenvironmental temperature range from 0 to 50 degrees Celsius, and bycompliance with the vibration, thermal shock, high temperature storageand thermal cycling test criteria in Telcordia GR-468-CORE andGR-1221-CORE. This target can be achieved by functional segmentation ofthe components and packaging the modular device with Telcordia-qualifiedpackaging technology. Before the modules are assembled into a system,they are tested and assembled separately.

Included in the modules are tap units that allow taking out signalsalong the propagation path in an integrated design. This is necessaryfor the optimization of each module as it is assembled, and important inthe spectral matching along the chain of modules.

Polarization units are provided to prevent the buildup of side-pulsesfrom orthogonal polarization light.

The acousto-optical down counter module can be designed to operate as abandwidth filter. For further modulation of the signal an additionalpulse extractor can be included near the end of the output. This unitsuffers from dispersion due to the large bandwidth of the pulse. Thecompressor can be used to correct for this dispersion as disclosedhereafter.

The invention also relates to a means to extract one or more chirpedpulses from a series of such pulses using an acousto-optic deflector,and to compensate for the detrimental effects on the spatialcharacteristics of the extracted chirped pulse caused by dispersion inthat deflector. An important aspect of this system is to manage thespectrum of the pulse in the system while maintaining the ability tocorrect for dispersion and compress the pulse back to the femtosecondregime. Two principal embodiments of this type will be described. Thefirst is the case where the spectral content of the seed pulse is small.In this case a nonlinear amplifier may be employed for the generation ofadditional spectrum while spectral filtering is employed to obtain acompressible pulse. The second case is where the spectrum from thesource is larger than necessary. Nonlinear affects can be limited in theamplifier chain in this case, while spectral filtering is again employedto obtain a compressible pulse. An additional attribute that isnecessary for many applications is the reduction of the ASE at theoutput. Specific amplifier designs are used to cut the ASE at the outputwavelength. The compressor can be used as an optical spectral filter tothis end.

Once gain performance is attained, a method for active stabilization ofthe optical performance of the gain fiber in a laser or amplifier isdisclosed to maintain this performance. The present invention stabilizesthe temperature dependent absorption of a gain fiber over a wideenvironmental temperature variation by an active feedback loop. A pieceof fiber, optically identical with the gain fiber itself, is used as aspectral filter for monitoring the emission spectrum of the pump diode.The absorption spectrum of the filter fiber follows that of the gainfiber if both fibers are packaged so that the fibers are in proximity toeach other. The transmission of the pump light through the filter fiberclones exactly the absorption characteristics of the gain fiber at agiven package temperature. The temperature of the pump diode iscontrolled by a feedback loop such that the transmission through thefilter fiber is maintained at the minimum. Importantly, the filter fiberfunctions as an active temperature sensor of the gain fiber. Precisespectral control of the gain at any fiber or package temperature canthus be realized.

As mentioned above, an important field of use for this system is inmicromachining. An additional feature needed for this application fieldis the capability to start and stop the pulse stream while moving thetargeted material in place. One method to do this is to control the downcounter. However, this leads to problems with gain stabilization in theamplifier and excessive ASE on target. These problems have beenaddressed in Ser. No. 10/813,173 “Method and Apparatus for Controllingand Protecting Pulsed High Power Fiber Amplifier Systems” (incorporatedby reference herein). However, another means to stop the pulse stream isto utilize an optical switch at the output.

The invention extracts one or more chirped pulses from a series of suchpulses using an acousto-optic deflector, and compensates for thedetrimental effects on the spatial characteristics of the extractedchirped pulse caused by dispersion in that deflector. The instantinvention has the additional advantage that the means to compensate fordispersion in the acousto-optic deflector can be used to compress theduration of the chirped pulse. This is accomplished by placing the AOMin proximity to a grating compressor.

Further improvements for correction of higher order dispersion terms infiber chirped pulse amplification systems are disclosed in AttorneyDocket No. A8717 (incorporated by reference herein). These can beapplied to chirped pulse amplification systems with regenerativeamplifiers.

Here, an ultra-compact high energy chirped pulse amplification systemsbased on linearly or nonlinearly chirped fiber grating pulse stretchersand photonic crystal fiber pulse compressors. Alternatively, photoniccrystal fiber pulse stretchers and photonic crystal fiber compressorscan also be implemented. For industrial applications the use ofall-fiber chirped pulse amplification systems is preferred, relying onfiber-based pulse compressors and stretchers as well as fiber-basedamplifiers.

Fiber-based high energy chirped pulse amplification systems of highutility can also be constructed from conventional optical componentssuch as pulse stretchers based on long lengths of conventional fiber aswell as bulk grating compressors. The performance of such ‘conventional’chirped pulse amplification systems can be greatly enhanced byexploiting nonlinear cubicon pulse formation, i.e. by minimization ofhigher-order dispersion via control of self-phase modulation inside theamplifiers.

Finally, a particularly compact seed source for an Yb fiber-basedchirped pulse amplification system can be constructed from ananti-Stokes frequency shifted modelocked Er fiber laser amplifiersystem, where a wavelength tunable output is obtained by filtering ofthe anti-Stokes frequency shifted output. The noise of such ananti-Stokes frequency shifted source is minimized by the amplificationof positively chirped pulses in a negative dispersion fiber amplifier.

The preceding improvements have been focused on systems operating closeto 1 μm. These systems appear to be the most suitable for industrialapplications. However, Ti:sapphire regenerative amplifiers are presentlythe dominant design. Frequency doubled erbium fiber lasers are utilizedfor the more industrial Ti:sapphire systems. FCPA front ends aresuitable for higher repetition rates utilizing an electro-optic pulseselector as is disclosed in Ser. No. 10/960,923. FCPA systems operatingin the 1.5 telecomm wavelength which are then frequency doubled would besuitable for a Ti:sapphire amplifier or regenerative amplifier system.

The invention in Ser. No. 10/606,829 (incorporated by reference herein)provides an erbium fiber (or erbium-ytterbium) based chirped pulseamplification system operating at a wavelength of approximately 1550nanometers. The use of fiber amplifiers operating in thetelecommunications window enables telecommunications components andtelecommunications compatible assembly procedures to be used, withsuperior mechanical stability

It is found that electronic controls are needed for reliable operationfor these complex systems. In Ser. No. 10/813,173 (incorporated byreference herein), the implementation of electronic controls aredescribed which prevent catastrophic damage in a short pulse amplifiersystem as well as maintaining constant output power over the life of thesystem. These systems are very applicable in a regenerative amplifiersystem seeded by a fiber laser. The damage issues will also be a concernin a regenerative amplifier system. However, more importantly thesefront end systems normally will encompass nonlinear optical processes inthe fiber amplifiers. These nonlinear optical processes are verydependent on laser intensity. Thus, to maintain the desired results overthe life of the system, careful control of the optical powers is neededparticularly in the nonlinear optical components in the system.

It is thus an object of the present invention to provide a high powerfiber amplifier system with means for controlling the pump diode currentand the gain of the fiber amplifier such that the output pulse energy isconstant as the pulse width and repetition rate are adjusted duringoperation. This includes keeping the pulse energy constant duringturn-on of the pulse train.

It is a further object of the invention to provide means for controllingthe temperature of the fiber amplifier pump diode such that the pumpdiode wavelength is maintained at a fixed value with changes in diodecurrent.

It is also an object of the invention to provide means for protectingthe high power amplifier from damage due to gain buildup in excess ofthe damage threshold of the amplifier by monitoring the repetition rateof the injected oscillator pulses or external signal, and shutting offor reducing the pump diode current if the repetition rate falls belowthis threshold.

It is also an object of the invention to provide for monitoring of theamplitude of the seed pulses and to protect the high power fiberamplifier from damage by shutting off the pump diode if the amplitude ofthe injected pulses falls outside a safe threshold.

It is also an object of the invention to provide a high power amplifiersystem with means for controlling the amplitude of the seed pulse suchthat the output energy of the power amplifier is constant.

The above and other objects of the invention are met by providing adevice and method for controlling the diode current of the pump diode ina high power fiber amplifier, the device comprising a means for settingthe pump diode current or power, monitoring such current or power, andmaintaining the diode current or power at a constant value. Typicallythe current of the diode is controlled to correct for long term decreaseon its output due to aging. In contrast, in accordance with anembodiment of the present invention, the pump diode current iscontrolled to dynamically control the gain of the power amplifier tomaintain uniform pulse energy as the repetition rate and the pulsetemporal width is changed. This includes turning the pump diode onsufficiently in advance and ramping up the current to produce equalpower for the first pulses when the unit is turned on.

The device also provides a means for calculating and/or storing thedesired pump diode current setting as a function of system pulse widthand repetition rate, such that the energy of the output pulse ismaintained at a desired value as the pulse width and repetition rate arevaried.

A device in accordance with an embodiment of the invention also providesa means for calculating and storing the appropriate pump diodetemperature setting as a function of the pump diode current setting,such that the emission wavelength of the pump diode is maintained at awavelength that provides maximum absorption of the pump diode energy bythe fiber amplifier medium as the pump diode current is varied.

Means are also provided to monitor the repetition rate of the injectedpulses into the amplifier system, to compare it to the predeterminedrepetition rate, and if lower than this repetition rate, to disable orreduce the current to the amplifier pump diode to prevent it from beingdamaged.

The exemplary device discussed above also provides a means for comparingthe amplitude of the pulse being injected into the fiber amplifier witha predetermined minimum amplitude value and if lower than thispredetermined minimum, a means to disable or reduce the current to theamplifier pump diode to prevent it from being damaged. A device inaccordance with an embodiment of the invention also provides a means ofselecting and attenuating the seed pulses such that the amplified outputpulses are of uniform energy.

It is an even further object of the invention to monitor the repetitionrate of the oscillator and to provide a means for calculating therequired down counter divide ratio needed to obtain a lower repetitionrate.

It is also an object of the invention to synchronize the oscillator withan external reference signal. It is also an object of the invention tovary this external reference in frequency, and have the oscillatorrepetition rate vary accordingly.

It is an even further object of the invention to vary the externalreference in frequency, and have the oscillator repetition rate varyaccordingly, and also have the down counted repetition rate varyaccordingly. However, this variation will be of a limited range comparedto an all fiber system due to the operation repetition rate of aregenerative amplifier.

Finally, these regenerative amplifier systems will be utilized in manycases for micromachining. Improvements for FPCA systems have beendeveloped that are unique for a fiber seed source. Ser. No. 10/813,389(incorporated by reference herein), describes the benefit for changingthe pulse shapes that allow the change of the material processingproperties of that laser. These methods include allowing the addition ofheat by the addition of longer pulses. The physical means for changingthese pulse shapes and building a all fiber chirped pulse amplificationsystem suitable for material processing is described Ser. No. 10/813,269(incorporated by reference herein). As is mentioned in Ser. No.10/813,269 some of these changes in the seed source for a fiber chirpedpulse amplification systems will also be suitable for regenerativeamplifier systems. Herein further methods of obtaining various pulsechanges are described.

The invention thus provides methods of materials processing using burstsof laser light comprised of ultrashort pulses in the femtosecond,picosecond and nanosecond ranges, wherein parameters of the pulsescomprising the burst, such as pulse width, pulse separation duration,pulse energy, wavelength and polarization, are manipulated to inducedesirable properties in the processed material.

While a precise and controlled removal of material is achieved usingultrashort pulses, there are situations when having a small amount ofthermal effect retained by the material from the previous pulse prior tobeing irradiated by a subsequent ultrashort pulse is beneficial. Inaddition, it is well known that the properties of most materials havesome dependence on temperature. For example, the absorption of light bysilicon is very dependent on temperature. Hence, heating such a targetmaterial can help initiate the ablation process at lower thresholdfluence and may produce a smoother surface. In general, the thermal andphysical effect or any change in structure caused by the prior pulseinfluences the laser matter interaction with the next pulse.

The ablation threshold energy density, as a function of pulse width, canvary significantly from the square root of t as pulse widths enter thefemtosecond range. These ultrashort pulses can be used to micro-machinecleanly without causing significant heat. These ultrashort pulses alsohave deterministic thresholds compared to the statistical thresholds oflonger pulses.

The present invention may be used in micro-machining with bursts ofpulses having pulse shapes that cannot be quantified by a single pulsewidth in order to describe their micro-machining properties. Forexample, a burst comprises a 100 femtosecond pulse and a one nanosecondpulse, where the one nanosecond pulse contains ninety percent of theenergy and the 100 femtosecond pulse contains ten percent of the energy.The threshold for ablation of gold is a little over 0.3 J/cm² for the100 femtosecond pulse and 3.0 J/cm² for the one nanosecond pulse. Thus,if the burst is focused to output 0.3 J/cm², then ablation will occurduring the 100 femtosecond pulse, and not during the one nanosecondpulse. If the one nanosecond pulse impinges upon the surface first, itwill have no affect while the 100 femtosecond pulse will ablate. Thus,the one nanosecond predominant pulse will not leave a heat affectedzone. However, if the 100 femtosecond pulse is right before the onenanosecond pulse, then the 100 femtosecond pulse will change theabsorption properties of the material so the one nanosecond pulse willalso interact with the material. In this case, the ablation processwould be predominantly heat related. If the one nanosecond pulse isincreased to 100 nanoseconds, then the pulse energy content in the longpulse can be increased by ten-fold but the threshold is still determinedby the ultrashort pulse and remains fixed even with one percent of thetotal energy in the ultrafast pulse.

Thus, in one embodiment of the present invention, the long pulse isbefore the ultrafast pulse if the pulse repetition rate is substantiallygreater than or equal 100 kilohertz. In another embodiment of thepresent invention, a portion of the long pulse follows after theultrafast pulse, and adding a pedestal on the short pulse can create thelong pulse. Micro-machining can be accomplished with an ultrashortpulse, where substantial energy is in a long pulse pedestal (>tenpicoseconds) and where the long pulse pedestal adds a thermal machiningmechanism.

The present invention can perform laser machining on material using aburst of ultrashort laser pulses and tailors the pulse width, pulseseparation duration, wavelength and polarization to maximize thepositive effect of thermal and physical changes achieved by the previouspulse on the laser matter interaction in a burst-machining mode. Betterprocessing results can be achieved by manipulating the pulse width, thepulse separation duration and the pulse energies of pulses within aburst. The wavelength and polarization of a laser beam also stronglyaffect the absorption of the laser beam, and have to be variedpulse-to-pulse in a burst in order to produce maximum laser-matterinteraction.

Besides the methods of manipulating laser beam parameters describedabove to achieve desired results, the present invention also includesmethods to achieve the thermal and physical enhancement of a materialduring laser processing. In an embodiment of the present invention, thebackground light (commonly referred to as Amplified Spontaneous Emission(ASE)) is controlled to provide a constant source of energy forachieving thermal and physical changes to enhance the machining byindividual ultrashort pulses. ASE is often emitted simultaneously andco-linearly with the ultrashort pulse from an amplified fiber laser.There are a number of ways to change the ASE ratio in the laser.Examples are changing the ultrashort pulse input energy into theamplifier, changing its center wavelength or changing the diode pumppower to the amplifier. Another means more variable is within thecompressor of the laser. As disclosed in application Ser. No. 10/813,163the spectral output of the ASE can be designed to be at a differentwavelength then that of the ultrashort pulse. Thus, in the compressor,where the spectral components are physically separated, a component canbe placed to block or partially block the ASE, as disclosed inapplication Ser. No. 10/813,163. The ratio between the ASE and theultrashort pulse energy can be controlled to vary the amount ofpreheating applied to the target material. In another embodiment of theinvention, a pedestal of an ultrashort pulse is controlled. The pedestalis similar to a superimposed long-pulse with lower amplitude.

The invention is based on the interaction with a material of laserpulses of different pulse widths, pulse separation duration, energy,wavelength and polarization in a burst mode. The positive aspects ofpulses having different pulse widths, pulse separation duration,energies, wavelengths and polarization are utilized, and a negativeaspect of one pulse complements a positive aspect of another pulse. Thecoupling of laser energy during interaction of successive laser pulseswith a material induces various thermal, physical and chemicalcouplings. The induced coupling involves microscopic change ofelectronic structure, phase transition, structural disintegration and/orother physical changes. For example, pulses with different pulse widthsin a burst induce coupling that is different from a burst having pulseswith the same pulse width.

An aspect of the invention provides a method of materials processingusing laser light. The method comprises applying bursts of laser lightto a target area of a material at a predetermined repetition rate.Preferably, the burst repetition rate is large enough for multipulsepulses generated within the round trip time of the regenerativeamplifier, although lower repetition rates can be used. The burst oflaser light comprises a first pulse and a second pulse of laser lightdisplaced in time, although more pulses could be used in the burst asnecessary. The first pulse has a first pulse width and the second pulsehas a second pulse width, and predetermined parameters of the firstpulse are selected to induce a change in a selected property of theprocessed material. The second pulse has a second pulse width, andpredetermined parameters of the second pulse are selected based upon theproperty change induced by the first pulse. The first pulse width isgenerally in the nanosecond range, and the second pulse width isgenerally in the picosecond to femtosecond range. However, as statedpreviously it can be reversed. Predetermined parameters include pulseenergy, pulse wavelength, pulse separation duration and pulsepolarization vector. These parameters of the first and second pulses arecontrolled as well to machine the target area of the processed material.

A still further aspect of the present invention provides a method ofmaterials processing that is similar to the previous aspect, except thatthe first and second pulses of the burst of laser light are overlappedin time, instead of being displaced in time. More pulses could be usedin the burst as necessary. The first pulse has a first pulse width andthe second pulse has a second pulse width, and the first pulse width canbe greater than the second pulse width. The first pulse has a firstpulse width and predetermined parameters of the first pulse are selectedto induce a change in a selected property of the processed material. Thesecond pulse has a second pulse width, and predetermined parameters ofthe second pulse are selected on based upon the property change inducedby the first pulse. The first pulse width is generally in the nanosecondrange, and the second pulse width is generally in the picosecond tofemtosecond range. Predetermined parameters include pulse energy, pulsewavelength, pulse separation duration and pulse polarization vectorwhich are controlled as well to machine the target area of the processedmaterial. In addition, the second pulse may include a pedestal tofacilitate thermally heating the processed material.

In yet another aspect of the present invention, an apparatus forgenerating optical pulses, wherein each pulse may have individualizedcharacteristics, is provided. The apparatus comprises a laser means forgenerating the bursts of pulses, a control means that controls the lasermeans and a beam manipulation means for monitoring the pulse width,wavelength, repetition rate, polarization and/or temporal delaycharacteristics of the pulses comprising the pulse bursts. The apparatusgenerates feedback data based on the measured pulse width, wavelength,repetition rate, polarization and/or temporal delay characteristics forthe control means. In one embodiment of the present invention, the lasermeans may comprise a fiber amplifier that uses stretcher gratings andcompressor gratings. The beam manipulation means can comprise a varietyof devices, e.g., an optical gating device that measures the pulseduration of the laser pulses, a power meter that measures the power ofthe laser pulses output from the laser means or a photodiode thatmeasures a repetition rate of the laser pulses. Another beammanipulation means optically converts the fundamental frequency of apercentage of the generated laser pulses to one or more other opticalfrequencies, and includes at least one optical member that converts aportion of the fundamental of the laser pulses into at least one higherorder harmonic signal. The optical member device may comprise anon-linear crystal device with a controller that controls the crystal'sorientation. Preferably, the means for converting an optical frequencyincludes a spectrometer that measures predetermined parameters of pulsesoutput from the non-linear crystal device and generates feedback for thecontrol means.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification illustrate embodiments of the invention and,together with the description, serve to explain the aspects, advantagesand principles of the invention. In the drawings,

FIG. 1 is a block diagram showing the basic components of the presentinvention.

FIG. 2 is an illustration of a modular, compact, tunable system forgenerating high peak and high average power ultrashort laser pulses inaccordance with the present invention;

FIG. 3 is an illustration of an embodiment of a Seed Module (SM) for usein the present invention;

FIG. 4 is a diagram graphically illustrating the relationship betweenthe average frequency-doubled power and wavelength which are output at agiven range of input power according to one embodiment of the presentinvention.

FIG. 5 is an illustration of an embodiment of a Pulse Compressor Module(PCM) for use with the present invention;

FIG. 6 is an illustration of an embodiment of a Pulse Stretcher Module(PSM) for use with the present invention;

FIG. 7 is an illustration of a second embodiment of a Seed Module (SM)for use with the present invention;

FIG. 8 is an illustration of a third embodiment of a Seed Module (SM)for use with the present invention;

FIG. 9 is an illustration of a fourth embodiment of a Seed Module (SM)for use with the present invention;

FIG. 10 is an illustration of a fifth embodiment of a Seed Module (SM)for use with the present invention;

FIG. 11 is an illustration of an embodiment of the present invention inwhich a Fiber Delivery Module (FDM) is added to the embodiment of theinvention shown in FIG. 1;

FIG. 12 is an illustration of an embodiment of a Fiber Delivery Module(FDM) for use with the present invention;

FIG. 13 is an illustration of a second embodiment of a Pulse StretcherModule (PSM) for use with the present invention;

FIG. 14 is an illustration of a third embodiment of a Pulse StretcherModule (PSM) for use with the present invention;

FIG. 15 is an illustration of an embodiment of the present invention inwhich pulse picking elements and additional amplification stages areadded.

FIG. 16 is an illustration of another embodiment of the presentinvention where a fiber amplifier is operated with at least one forwardand one backward pass, in combination with optical modulators such aspulse picking elements.

FIG. 17 is a diagram of a cladding pumped fiber cavity design accordingto a first embodiment of the invention.

FIG. 18 a is a diagram of a saturable absorber mirror according to anembodiment of the invention.

FIG. 18 b is a diagram of a saturable absorber mirror according to analternative embodiment of the invention.

FIG. 19 is a diagram of the proton concentration as a function of depthobtained after proton implantation into a saturable semiconductor film.

FIG. 20 is a diagram of the measured bi-temporal reflectivity modulationobtained in a semiconductor saturable mirror produced byion-implantation with selective depth penetration.

FIG. 21 a is a diagram of a scheme for coupling a saturable absorbermirror to a fiber end according to an embodiment of the invention.

FIG. 21 b is a diagram of a scheme for coupling a saturable absorbermirror to a fiber end according to an alternative embodiment of theinvention.

FIG. 22 is a diagram for increasing the optical bandwidth of a fiberlaser according to an embodiment of the invention.

FIG. 23 is a diagram of a core pumped fiber cavity design according toan embodiment of the invention.

FIG. 24 is a diagram of a core pumped fiber cavity design usingintra-cavity wavelength division multiplexers and output couplersaccording to an embodiment of the invention.

FIG. 25 is a diagram of a core pumped fiber cavity design usingintra-cavity wavelength division multiplexers and a butt-coupled fiberpig-tail for output coupling according to an embodiment of theinvention.

FIG. 26 is a diagram of a cladding pumped fiber cavity design using anintra-cavity output coupler according to an embodiment of the invention.

FIG. 27 is a diagram of a cladding pumped fiber cavity design usingintra-cavity fiber output couplers according to an embodiment of theinvention.

FIG. 28 a is a diagram of a passively modelocked fiber laser based onconcatenated sections of polarization maintaining and non-polarizationmaintaining fiber sections according to an embodiment of this invention.

FIG. 28 b is a diagram of a passively modelocked fiber laser based onconcatenated sections of long polarization maintaining fiber sectionsaccording to an embodiment of this invention.

FIG. 28 c is a diagram of a passively modelocked fiber laser based onshort concatenated sections of polarization maintaining fiber andadditional sections of all-fiber polarizer according to an embodiment ofthis invention.

FIG. 29 is a diagram of a dispersion compensated fiber laser cavityaccording to an embodiment of this invention.

FIG. 30 is a diagram of a dispersion compensated fiber laser cavityaccording to an alternative embodiment of this invention, includingmeans for additional spectral broadening of the fiber laser output.

FIG. 31 is a diagram of a design based on a fiber based MOPA having thefewest bulk optical components, according to a further embodiment.

FIG. 32 is an embodiment which includes monitoring electronics andfeedback control of a fiber based pulse source.

FIG. 33 a illustrates a module usable for polarization correction or asvariable attenuation in a fiber based laser system.

FIG. 33 b illustrates a particularly preferred embodiment for a fibersolid-state regenerative amplifier system.

FIG. 34 shows a source of ultra-fast pulses based upon a microchiplaser.

FIG. 35 illustrates a source based on a DFB laser and a lithium niobatepulse generator.

FIG. 36 illustrates a system allowing independent control of higherorder dispersion and self-phase modulation.

FIG. 37 illustrates an algorithm for a control system for ensuringmode-locking

FIG. 38 illustrates an embodiment enabling the gain bandwidth of theregenerative amplifier to be easily matched to the fiber amplifiersystem.

FIG. 39 illustrates a generic scheme for the amplification of the outputof a FCPA system in a bulk optical amplifier.

FIG. 40 illustrates an embodiment employing a series of chirped gratingsoperating on different portions of the spectrum, for elongating thepulse envelope.

FIGS. 41 and 42 show a laser diode-based multiple pulse source, and alaser system including this source.

FIGS. 43 a-43 c show outputs of the pulse source of FIG. 41 in graphicform.

FIG. 44 illustrates a wavelength router scheme usable with theembodiment of FIG. 41; and

FIG. 45 illustrates a fiber splitter arrangement useable in theembodiment of FIG. 41.

DETAILED DESCRIPTION OF THE INVENTION

A generalized illustration of the system of the invention is shown inFIG. 1. The pulses are generated in a short pulse source. 11. These arecoupled into a pulse conditioner 12 for spectral narrowing, broadeningor shaping, wavelength converting, temporal pulse compression orstretching, pulse attenuation and/or lowering the repetition rate of thepulse train. The pulses are subsequently coupled into an Yb: or Nd:fiber amplifier 13. Pulse stretcher 14 provides further pulse stretchingbefore the amplification in the regenerative amplifier 15 that is basedon an Nd: or Yb: doped solid-state laser material. The compressor 16compresses the pulse back to near transform limit. The six basicsubsystems described here are each subject to various implementations,as is described in the subsequent embodiments.

A generalized illustration of one embodiment of the short pulse source11 is shown in FIG. 2. The pulses generated in a laser seed source 1(seed module; SM) are coupled into a pulse stretcher module 2 (PSM),where they are dispersively stretched in time. The stretched pulses aresubsequently coupled into the fundamental mode of a cladding-pumped Ybfiber amplifier 3 (amplifier module, AM1), where the pulses areamplified by at least a factor of 10. Finally, the pulses are coupledinto a pulse compressor module 4 (PCM), where they are temporallycompressed back to approximately the bandwidth limit.

The embodiment shown in FIG. 2 is modular and four sub-systems; the SM1, PSM 2, AM1 3 and PCM 4. The sub-systems can be used independently aswell as in different configurations, as described in the alternativeembodiments.

In the following, discussion is restricted to the SM-PSM-AM1-PCM system.The SM 1 preferably comprises a femtosecond pulse source (seed source5). The PSM preferably comprises a length of fiber 6, where couplingbetween the SM and the PSM is preferably obtained by fusion splicing.The output of the PSM is preferably injected into the fundamental modeof the Yb amplifier 7 inside the AM1 module 3. Coupling can be performedby fusion splicing, a fiber coupler or a bulk-optic imaging systembetween PSM 2 and the fiber amplifier 7. All fibers are preferablyselected to be polarization maintaining The PCM 4 is preferably adispersive delay line constructed from one or two bulk optic diffractiongratings for reasons of compactness. Alternatively, a number of bulkoptic prisms and Bragg gratings can be used inside the PCM 4. Couplingto the PCM 4 can be performed by a bulk optic lens system as representedby the single lens 8 in FIG. 2. In the case of a PCM that contains fiberBragg gratings, a fiber pig-tail can be used for coupling to the PCM.

As an example of a femtosecond laser seed source, a Raman-shifted,frequency-doubled Er fiber laser is shown within an SM 1 b in FIG. 3.The femtosecond fiber laser 9 can be a commercial high energy solitonsource (IMRA America, Inc., Femtolite B60™) delivering ≈200 fs pulses ata wavelength of 1.57 μm and a pulse energy of 1 nJ at a repetition rateof 50 MHz.

For optimum Raman-shifting from 1.5 μm to the 2.1 μm wavelength region,a reduction in the core diameter (tapering) along the length of thepolarization maintaining Raman-shifting fiber 10 is introduced. Areduction of the core diameter is required to keep the 2nd orderdispersion in the Raman-shifter close to zero (but negative) in thewhole wavelength range from 1.5 to 2.1 μm. By keeping the absolute valueof the 2nd order dispersion small, the pulse width inside the Ramanshifter is minimized, which leads to a maximization of the Ramanfrequency shift (J. P. Gordon, “Theory of the Soliton Self-frequencyShift,” Opt. Lett., 11, 662 (1986)). Without tapering, the Ramanfrequency-shift is typically limited to around 2.00 μm, which even afterfrequency-doubling is not compatible with the gain bandwidth of Yb fiberamplifiers.

In this particular example, a two-stage Raman shifter 10 consisting of30 and 3 m lengths of silica ‘Raman’ fiber (single-mode at 1.56 μm) withcore diameters of 6 and 4 μm respectively, was implemented. Due to theonset of the infrared absorption edge of silica at 2.0 μm, it isbeneficial to increase the rate of tapering towards the end of the Ramanshifter 10. In the present example, conversion efficiencies up to 25%from 1.57 μm to 2.10 μm were obtained. Even better conversionefficiencies can be obtained by using a larger number of fibers withsmoothly varying core diameter, or by implementing a single taperedfiber with smoothly varying core diameter.

Frequency-conversion of the Raman-shifted pulses to the 1.05 μm regioncan be performed by a length of periodically poled LiNbO3 (PPLN) crystal11 with an appropriately selected poling period. (Although throughoutthis specification, the preferable material for frequency conversion isindicated as PPLN, it should be understood that other periodically-poledferroelectric optical materials such as PP lithium tantalate, PPMgO:LiNbO₃, PP KTP, or other periodically poled crystals of the KTPisomorph family can also be advantageously used.) Coupling with the PPLNcrystal 11 occurs through the use of a lens system, represented in FIG.3 by lenses 12. The output of the PPLN crystal 11 is coupled by lenses12 into output fiber 13. Conversion efficiencies as high as 16% can sobe obtained for frequency-doubling of 2.1 μm resulting in a pulse energyup to 40 pJ in the 1 μm wavelength region. The spectral width of thefrequency-converted pulses can be selected by an appropriate choice ofthe length of the PPLN crystal 11; for example a 13 mm long PPLN crystalproduces a bandwidth of 2 nm in the 1.05 μm region corresponding to apulse width of around 800 fs. The generated pulse width is approximatelyproportional to the PPLN crystal length, i.e., a frequency convertedpulse with a 400 fs pulse width requires a PPLN length of 6.5 mm. Thispulse width scaling can be continued until the frequency-converted pulsewidth reaches around 100 fs, where the limited pulse width of 100 fs ofthe Raman-shifted pulses limits further pulse width reduction.

In addition, when the frequency-converted pulse width is substantiallylonger than the pulse width of the Raman-shifted pulses, the widebandwidth of the Raman-pulses can be exploited to allow forwavelength-tuning of the frequency-converted pulses, i.e., efficientfrequency conversion can be obtained for pulses ranging in frequencyfrom 2(ω₁−δω) to 2(ω₁+δω), where 2δω is the spectral width at halfmaximum of the spectrum of the Raman-shifted pulses. Continuouswavelength tuning here is simply performed by tuning the temperature ofthe frequency-conversion crystal 11.

The amplified noise of the Raman-shifter, PPLN-crystal combination isminimized as follows. Self-limiting Raman-shifting of the Er fiber laserpulse source can be used by extending the Raman shift out to larger than2 μm in silica-based optical fiber. For wavelengths longer than 2 μm,the infrared absorption edge of silica starts to significantly attenuatethe pulses, leading to a limitation of the Raman shift and a reductionin amplitude fluctuations, i.e., any increase in pulse energy at 1.5 μmtends to translate to a larger Raman-shift and thus to a greaterabsorption in the 2 μm wavelength region, which thus stabilizes theamplitude of the Raman-shifted pulses in this region.

Alternatively, the noise of the nonlinear frequency conversion processcan be minimized by implementing self-limiting frequency-doubling, wherethe center wavelength of the tuning curve of the doubling crystal isshorter than the center wavelength of the Raman-shifted pulses. Again,any increase in pulse energy in the 1.5 μm region translates into alarger Raman-shift, producing a reduced frequency conversion efficiency,and thus the amplitude of the frequency-doubled pulses is stabilized.Therefore a constant frequency-converted power can be obtained for alarge variation in input power.

This is illustrated in FIG. 4, where the average frequency-convertedpower in the 1 μm wavelength region as a function of average input powerat 1.56 μm is shown. Self-limiting frequency-doubling also ensures thatthe frequency-shifted wavelength in the 1 μm wavelength region isindependent of average input power in the 1.56 μm wavelength region, asalso demonstrated in FIG. 4.

Several options exist for the PSM 2. When a length of fiber 6(stretching fiber) is used as a PSM as shown in FIG. 2, an appropriatedispersive delay line can then be used in the PCM 4 to obtain nearbandwidth-limited pulses from the system. However, when the dispersivedelay line in the PCM 4 consists of bulk diffraction gratings 14 asshown in FIG. 5, a possible problem arises. The ratio of|3^(rd)/2^(nd)|-order dispersion is typically 1-30 times larger indiffraction grating based dispersive delay lines compared to the ratioof |3^(rd)/2^(nd)|-order dispersion in typical step-index optical fibersoperating in the 1 μm wavelength region. Moreover, for standardstep-index fibers with low numerical apertures operating in the 1 μmwavelength regime, the sign of the third-order dispersion in the fiberis the same as in a grating based dispersive delay line. Thus a fiberstretcher in conjunction with a grating-based stretcher does nottypically provide for the compensation of 3^(rd)- and higher-orderdispersion in the system.

For pulse stretching by more than a factor of 10, the control ofthird-order and higher-order dispersion becomes important for optimalpulse compression in the PCM 4. To overcome this problem, the stretcherfiber 6 in the PSM 2 can be replaced with a length of fibers withW-style multi-clad refractive index profiles, i.e., ‘W-fibers’ (B. J.Ainslie et al.) or holey fibers (T. M. Monroe et al., ‘Holey OpticalFibers’ An Efficient Modal Model, J. Lightw. Techn., vol. 17, no. 6, pp.1093-1102). Both W-fibers and holey fibers allow adjustable values of2nd, 3rd and higher-order dispersion. Due to the small core sizepossible in W and holey fibers, larger values of 3rd order dispersionthan in standard single-mode fibers can be obtained. The implementationis similar to the one shown in FIG. 1 and is not separately displayed.The advantage of such systems is that the PSM can work purely intransmission, i.e., it avoids the use of dispersive Bragg gratingsoperating in reflection, and can be spliced into and out of the systemfor different system configurations.

An alternative PSM 2 with adjustable values of 2^(nd), 3^(rd) and 4^(th)order dispersion is shown in FIG. 6. The PSM 20 a is based on theprinciple that conventional step-index optical fibers can produce eitherpositive, zero or negative 3rd order dispersion. The highest amount of3rd order dispersion in a fiber is produced by using its firsthigher-order mode, the LP₁₁ mode near cut-off. In FIG. 6, the 4^(th) and3^(rd) order dispersion of the PSM 20 a is adjusted by using threesections 15, 16, 17 of pulse stretching fiber. The 1st stretcher fiber15 can be a length of fiber with zero 3rd-order and appropriate4^(th)-order dispersion. The 1st stretcher fiber 15 is then spliced tothe 2^(nd) stretcher fiber 16, which is selected to compensate for the3^(rd)-order dispersion of the grating compressor as well as the wholechirped-pulse amplification system. To take advantage of the high3^(rd)-order dispersion of the LP₁₁ mode the 1st stretcher fiber 15 isspliced to the 2^(nd) stretcher fiber 16 with an offset in theirrespective fiber centers, leading to a predominant excitation of theLP₁₁ mode in the 2nd stretcher fiber 16. To maximize the amount of3rd-order dispersion in the 2nd stretcher fiber 16, a fiber with a highnumerical aperture NA>0.20 is preferred. At the end of the 2nd stretcherfiber 16, a similar splicing technique is used to transfer the LP₁₁ modeback to the fundamental mode of the 3^(rd) stretcher fiber 17. By anappropriate choice of fibers, the 4th-order dispersion of the wholeamplifier compressor can be minimized. The 3^(rd) stretcher fiber 17 canbe short with negligible dispersion.

The transfer loss of the whole fiber stretcher assembly is at least 25%due to the unavoidable 50% or greater loss incurred by transferringpower from the LP₁₁ mode to the LP₀₁ mode without the use of opticalmode-converters. Any residual energy in the LP₀₁ mode in the 2ndstretcher fiber can be reflected with an optional reflective fibergrating 18 as shown in FIG. 6. Due to the large difference in effectiveindex between the fundamental and the next higher-order mode, thegrating resonance wavelength varies between 10-40 nm between the twomodes, allowing for selective rejection of one mode versus the other forpulses with spectral widths between 10-40 nm.

The energy loss of the fiber stretcher assembly can be made to beinsignificant by turning the 3^(rd) stretcher fiber 17 into an Ybamplifier. This implementation is not separately shown.

When 4th-order dispersion is not significant, the 1st stretcher fiber 15can be omitted. 4^(th) order dispersion can also be compensated by usinga 1st stretcher fiber with non-zero 3^(rd) order dispersion, as long asthe ratio of 3^(rd) and 4^(th) order dispersion is different between the1^(st) and 2^(nd) stretcher fiber.

The Yb-doped fiber inside the AM1 3 can have an Yb doping level of 2.5mole % and a length of 5 m. Both single-mode and multi-mode Yb-dopedfiber can be used, where the core diameter of the fiber can vary between1-50 μm; though the fundamental mode should be excited in case of a MMfiber to optimize the spatial quality of the output beam. Depending onthe amount of required gain, different lengths of Yb-doped fiber can beused. To generate the highest possible pulse energies, Yb fiber lengthsas short as 1 m can be implemented.

Pulse compression is performed in the PCM 4. The PCM 4 can containconventional bulk optic components (such as the bulk diffraction gratingpair shown in FIG. 5), a single grating compressor, or a number ofdispersive prisms or grisms or any other dispersive delay line.

Alternatively, a fiber or bulk Bragg grating can be used, or a chirpedperiodically poled crystal. The chirped periodically poled crystalcombines the functions of pulse compression and frequency doubling (A.Galvanauskas, et al., ‘Use of chirped quasi-phase matched materials inchirped pulse amplification systems,’ U.S. application Ser. No.08/822,967, the contents of which are hereby incorporated herein byreference) and operates in transmission providing for a uniquely compactsystem.

Other modifications and variations to the invention will be apparent tothose skilled in the art from the foregoing disclosure and teachings.

In particular, the SM 1 can be used as a stand-alone unit to producenear bandwidth limited femtosecond pulses in the frequency range from1.52-2.2 μm, and after frequency conversion in a nonlinear crystal alsoin the frequency range from 760 nm to 1.1 μm. The frequency range can befurther extended by using a fluoride Raman-shifting fiber or otheroptical fibers with infrared absorption edges longer than silica. Usingthis technique wavelengths up to around 3-5 μm can be reached. Inconjunction with frequency-doubling, continuous tuning from 760 nm to5000 nm can be achieved. The pulse power in the 2 μm region can befurther enhanced by using Tm or Ho-doped fiber. With such amplifiers,near bandwidth-limited Raman-soliton pulses with pulse energiesexceeding 10 nJ can be reached in single-mode fibers in the 2 μmwavelength region. After frequency-doubling, femtosecond pulses withenergies of several nJ can be obtained in the 1 μm region without theuse of any dispersive pulse compressors. Such pulses can be used as highenergy seed pulses for large-core multi-mode Yb amplifiers, whichrequire higher seed pulse energies than single-mode Yb amplifiers tosuppress amplified spontaneous emission.

An example of an ultra-wide tunable fiber source combining an Er-fiberlaser pulse source 19 with a silica Raman-shifter 20, a Tm-dopedamplifier 21 and a 2^(nd) fluoride glass based Raman shifter 22 is shownin the SM 1 c of FIG. 7. An optional frequency-doubler is not shown forconverting into the 900 nm to 1050 nm range. This would be a means forobtaining a high power source in this range. For optimum stability allfibers should be polarization maintaining. As another alternative to theEr-fiber laser pulse source a combination of a diode-laser pulse sourcewith an Er-amplifier can be used; this is not separately shown.

As yet another alternative for a SM, SM 1 d is shown in FIG. 8, andcontains a frequency-doubled high-power passively mode-locked Er orEr/Yb-fiber oscillator 23 in conjunction with a length of Raman-shiftingholey fiber 24. Here the pulses from the oscillator 23 operating in the1.55 μm wavelength region are first frequency-doubled using frequencydoubler 25 and lens system 26, and subsequently the frequency-doubledpulses are Raman-shifted in a length of holey fiber 24 that providessoliton supporting dispersion for wavelengths longer than 750 nm or atleast longer than 810 nm. By amplifying the Raman-shifted pulses in the1 μm wavelength regime or in the 1.3, 1.5, or 2 μm wavelength regime andby selecting different designs of Raman-shifting fibers, a continuouslytunable source operating in the wavelength region from around 750 nm to5000 nm can be constructed. The design of such a source with a number ofattached amplifiers 27 is also shown in FIG. 8.

For optimum Raman self-frequency shift, the holey fiber dispersionshould be optimized as a function of wavelength. The absolute value ofthe 3rd order dispersion of the holey fiber should be less than or equalto the absolute value of the 3rd order material dispersion of silica.This will help ensure that the absolute value of the 2nd orderdispersion remains small over a substantial portion of the wavelengthtuning range. Moreover the value of the 2nd order dispersion should benegative, and a 2nd order dispersion zero should be within 300 nm inwavelength to the seed input wavelength.

As yet another alternative for a seed source for an Yb amplifier,anti-Stokes generation in a length of anti-Stokes fiber can be used.After anti-Stokes generation, additional lengths of fiber amplifiers andRaman-shifters can be used to construct a widely wavelength-tunablesource. A generic configuration is similar to the one shown in FIG. 8,where the frequency-doubling means 25 are omitted and the Raman-shiftermeans 24 are replaced with an anti-Stokes generation means. For example,to effectively generate light in the 1.05 μm wavelength regime in ananti-Stokes generation means using an Er fiber laser seed sourceoperating at 1.55 μm, an anti-Stokes generation means in the form of anoptical fiber with small core diameter and a low value of 3^(rd) orderdispersion is optimum. A low value of 3^(rd) order dispersion is heredefined as a value of 3^(rd) order dispersion smaller in comparison tothe value of 3^(rd) order dispersion in a standard telecommunicationfiber for the 1.55 wavelength region. Moreover, the value of the 2^(nd)order dispersion in the anti-Stokes fiber should be negative.

As yet another alternative seed-source for an Yb amplifier, a passivelymodelocked Yb or Nd fiber laser can be used inside the SM. Preferably anYb soliton oscillator operating in the negative dispersion regime can beused. To construct an Yb soliton oscillator, negative cavity dispersioncan be introduced into the cavity by an appropriately chirped fibergrating 29, which is connected to output fiber 36 as shown in FIG. 9;alternatively, negative dispersion fiber such as holey fiber (T. Monroeet al.) can be used in the Yb soliton laser cavity. A SM incorporatingsuch an arrangement is shown as SM 1 e in FIG. 9. Here the Yb fiber 30can be polarization maintaining and a polarizer 31 can be incorporatedto select oscillation along one axis of the fiber (coupling beingaccomplished with lenses 32). For simplicity, the Yb fiber 30 can becladding pumped from the side as shown in FIG. 9. However, a passivelymodelocked Yb fiber laser incorporating conventional single-mode fiberwith conventional pumping through a WDM can also be used. Such anarrangement is not separately shown. In FIG. 9, SA 28 is used to inducethe formation of short optical pulses. The grating 35 is used fordispersive control, and as an intra-cavity mirror. The pump diode 33delivers pump light through V-groove 34.

An arrangement incorporating a holey fiber can be nearly identical tothe system displayed in FIG. 9, where an additional length of holeyfiber is spliced anywhere into the cavity. In the case of incorporatinga holey fiber, the fiber Bragg grating does not need to have negativedispersion; equally the Bragg grating can be replaced with a dielectricmirror.

Most straight-forward to implement, however, is an Yb oscillatoroperating in the positive dispersion regime, which does not require anyspecial cavity components such as negative dispersion fiber Bragggratings or holey fiber to control the cavity dispersion. In conjunctionwith a ‘parabolic’ Yb amplifier (or ordinary Yb amplifier), a verycompact seed source for a high-power Yb amplifier system can beobtained. Such a Yb oscillator with an Yb amplifier 40 is shown in FIG.10, where preferably the Yb amplifier 40 is a ‘parabolic’ Yb amplifieras discussed below. Elements which are identical to those in FIG. 9 areidentically numbered.

The SM 1 f in FIG. 10 comprises a side-pumped Yb amplifier 40 asdescribed with respect to FIG. 9, though any other pumping arrangementcould also be implemented. The Yb fiber 44 is assumed to be polarizationmaintaining and a polarizer 31 is inserted to select a singlepolarization state. The fiber Bragg grating 37 has a reflectionbandwidth small compared to the gain bandwidth of Yb and ensures theoscillation of pulses with a bandwidth small compared to the gainbandwidth of Yb. The Bragg grating 37 can be chirped or unchirped. Inthe case of an unchirped Bragg grating, the pulses oscillating insidethe Yb oscillator are positively chirped. Pulse generation or passivemodelocking inside the Yb oscillator is initiated by the saturableabsorber 28. The optical filter 39 is optional and further restricts thebandwidth of the pulses launched into the Yb amplifier 40.

To optimize the formation of parabolic pulses inside the Yb amplifier 40inside the SM 1 f, the input pulses should have a bandwidth smallcompared to the gain bandwidth of Yb; also the input pulse width to theYb amplifier 40 should be small compared to the output pulse width andthe gain of the Yb amplifier 40 should be as high as possible, i.e.,larger than 10. Also, gain saturation inside the Yb amplifier 40 shouldbe small.

As an example of a parabolic amplifier a Yb amplifier of 5 m in lengthcan be used. Parabolic pulse formation is ensured by using a seed sourcewith a pulse width of around 0.2-1 ps and a spectral bandwidth on theorder of 3-8 nm. Parabolic pulse formation broadens the bandwidth of theseed source to around 20-30 nm inside the Yb amplifier 40, whereas theoutput pulses are broadened to around 2-3 ps. Since the chirp insideparabolic pulses is highly linear, after-compression pulse widths on theorder of 100 fs can be obtained. Whereas standard ultrafast solid stateamplifiers can tolerate a nonlinear phase shift from self-phasemodulation only as large as pi (as well known in the state of the art),a parabolic pulse fiber amplifier can tolerate a nonlinear phase shiftas large as 10*pi and higher. For simplicity, we thus refer to a largegain Yb amplifier as a parabolic amplifier. Parabolic amplifiers obeysimple scaling laws and allow for the generation of parabolic pulseswith spectral bandwidths as small as 1 nm or smaller by an appropriateincrease of the amplifier length. For example, a parabolic pulse with aspectral bandwidth of around 2 nm can be generated using a parabolicamplifier length of around 100 m.

Since a parabolic pulse can tolerate large values of self-modulation anda large amount of spectral broadening without incurring any pulse breakup, the peak power capability of a parabolic amplifier can be greatlyenhanced compared to a standard amplifier. This may be explained asfollows. The time dependent phase delay Φ_(nl)(t) incurred by self-phasemodulation in an optical fiber of length L is proportional to peakpower, i.e.

Φ_(nl)(t)=γP(t)L,

where P(t) is the time dependent peak power inside the optical pulse.The frequency modulation is given by the derivative of the phasemodulation, i.e., δω=γL[∂P(t)/∂t]. For a pulse with a parabolic pulseprofile P(t)=P₀[1−(t/t₀)²], where (−t₀<t<t₀), the frequency modulationis linear. It may then be shown that indeed the pulse profile also staysparabolic, thus allowing the propagation of large peak powers with onlya resultant linear frequency modulation and the generation of a linearpulse chirp.

The chirped pulses generated with the Yb amplifier 40 can be compressedusing a diffraction grating compressor as shown in FIG. 5.Alternatively, the pulses can be left chirped and compensated with thecompressor after the regenerative amplifier.

In addition to the passively modelocked Yb fiber laser 44 shown in FIG.10, alternative sources could also be used to seed the Yb amplifier.These alternative sources can comprise Raman-shifted Er or Er/Yb fiberlasers, frequency-shifted Tm or Ho fiber lasers and also diode laserpulse sources. These alternative implementations are not separatelyshown.

In FIG. 11 a fiber delivery module (FDM) 45 is added to the basic systemshown in FIG. 2. The PSM 2 is omitted in this case; however, to expandthe peak power capability of the amplifier module a PSM 2 can beincluded when required. The Yb amplifier 7 shown in FIG. 11 can beoperated both in the non-parabolic or the parabolic regime.

In its simplest configuration, the FDM 45 consists of a length ofoptical fiber 46 (the delivery fiber). For a parabolic amplifier, thedelivery fiber 46 can be directly spliced to the Yb amplifier 7 withoutincurring any loss in pulse quality. Rather, due to the parabolic pulseprofile, even for large amounts of self-phase modulation, anapproximately linear chirp is added to the pulse allowing for furtherpulse compression with the PCM 4. The PCM 4 can be integrated with theFDM 45 by using a small-size version of the bulk diffraction gratingcompressor 14 shown in FIG. 5 in conjunction with a delivery fiber. Inthis case the delivery fiber in conjunction with an appropriatecollimating lens would replace the input shown in FIG. 5. A separatedrawing of such an implementation is not shown. However, the use of thePCM 4 is optional and can for example be omitted, if chirped outputpulses are required from the system. In conjunction with a PCM 4, thesystem described in FIG. 11 constitutes a derivative of a chirped pulseamplification system, where self-phase modulation as well as gain isadded while the pulse is dispersively broadened in time. The addition ofself-phase modulation in conventional chirped pulse amplificationsystems typically leads to significant pulse distortions after pulsecompression. The use of parabolic pulses overcomes this limitation.

To obtain pulse widths shorter than 50 fs, the control of third orderand higher-order dispersion in a FDM module or in an optional PSMbecomes significant. The control of higher-order dispersion with a PSMwas already discussed with reference to FIGS. 2 and 6; the control ofhigher-order dispersion in a FDM is very similar and discussed with anexemplary embodiment of the FDM 45 a shown in FIG. 12. Just as in FIG.2, the large third-order dispersion of a W-fiber can be used tocompensate for the third-order dispersion of a bulk PCM 4. Just as inFIG. 6, by using fibers 15, 16, 17 with different values forhigher-order dispersion in the FDM, the higher order dispersion of thewhole system including a PCM 4 consisting of bulk diffraction gratingsmay be compensated.

Alternative embodiments of PSMs are shown in FIGS. 13 and 14, which arealso of practical value as they allow the use of commercially availablelinearly chirped fiber Bragg gratings in the PSM, while compensating forhigher-order dispersion of a whole chirped-pulse amplification systemcomprising PSM as well as PCM. As another alternative, nonlinearlychirped fiber Bragg gratings can also be used in the PSM to compensatefor the dispersion of the PCM. Such an arrangement is not separatelyshown.

Alternatively, the pulses can be left chirped and compensated with thecompressor after the regenerative amplifier. This would mean notutilizing the PCM. This design would place additional design challengeson the dispersion correction in the PSM.

To avoid the use of W-fibers or the LP₁₁ mode in the PSM, an alternativeembodiment of a PSM as shown in FIG. 13 is shown as PSM 2 b. Here anegatively linearly chirped Bragg grating 47 is used in conjunction witha single-mode stretcher fiber 48 with negative third-order dispersionand circulator 49. The introduction of the negative linearly chirpedBragg grating increases the ratio of (3^(rd)/2^(nd))-order dispersion inthe PSM 2 b, allowing for the compensation of the high value of 3^(rd)order dispersion in the PCM 4, when a bulk diffraction gratingcompressor is used. The PSM 2 b can also contain W-fibers in conjunctionwith a linearly chirped fiber Bragg grating to further improve theflexibility of the PSM.

As yet another alternative embodiment of a PSM for the compensation ofhigher-order dispersion the arrangement in FIG. 14 is shown as PSM 2 c,comprising a positively linearly chirped fiber Bragg grating 50,circulator 49 and another fiber transmission grating 51. Here thepositively linearly chirped fiber Bragg grating 50 produces positive 2ndorder dispersion and the other fiber transmission grating 51 produces anappropriate amount of additional 2^(nd) 3^(rd) and 4^(th) orderdispersion, to compensate for the linear and higher order dispersioninside the PCM module. More than one fiber transmission grating or fiberBragg grating can be used to obtain the appropriate value of 3^(rd) and4^(th) and possibly even higher-order dispersion.

To increase the amplified pulse energy from an Yb amplifier to higherpulse energies, pulse picking elements and further amplification stagescan be implemented as shown in FIG. 15. In this case, pulse pickers 52are inserted in between the PSM 2 and the 1^(st) amplifier module AM1 3a, as well as between the 1st amplifier stage AM1 3 a and 2^(nd)amplifier stage AM2 3 b. Any number of amplifiers and pulse pickers canbe used to obtain the highest possible output powers, where the finalamplifier stages preferably consist of multi-mode fibers. To obtain adiffraction limited output the fundamental mode in these multi-modeamplifiers is selectively excited and guided using well-known techniques(M. E. Fermann et al., U.S. Pat. No. 5,818,630 and U.S. application Ser.No. 10/424,220) (both incorporated by reference herein). The pulsepickers 52 are typically chosen to consist of optical modulators such asacousto-optic or electro-optic modulators. The pulse pickers 52down-count the repetition rate of the pulses emerging from the SM 1 by agiven value (e.g. from 50 MHz to 5 KHz), and thus allow the generationof very high pulse energies while the average power remains small.Alternatively, directly switchable semiconductor lasers could also beused to fix the repetition rate of the system at an arbitrary value.Further, the pulse pickers 52 inserted in later amplifier stages alsosuppress the build up of amplified spontaneous emission in theamplifiers allowing for a concentration of the output power inhigh-energy ultra-short pulses. The amplification stages are compatiblewith PSMs and PCMs as discussed before; where the dispersion of thewhole system can be minimized to obtain the shortest possible pulses atthe output of the system.

Amplifier module AM1 3 a can be designed as a parabolic amplifierproducing pulses with a parabolic spectrum. Equally, the parabolicpulses from AM1 3 a can be transformed into pulses with a parabolicpulse spectrum in a subsequent length of pulse-shaping or pulsestretching fiber 53 as also shown in FIG. 15, where the interaction ofself-phase modulation and positive dispersion performs thistransformation. This may be understood, since a chirped pulse with aparabolic pulse profile can evolve asymptotically into a parabolic pulsewith a parabolic spectrum in a length of fiber. The parabolic pulseshape maximizes the amount of tolerable self-phase modulation in thesubsequent amplification stages, which in turn minimizes the amount ofdispersive pulse stretching and compression required in the PSM 2 andPCM 4. Equally, parabolic pulse shapes allow the toleration ofsignificant amounts of self-phase modulation in the PSM 2 withoutsignificant pulse distortions.

Once the pulses are stretched, the detrimental influence of self-phasemodulation in subsequent amplifiers can be minimized by using flat-toppulse shapes. A flat-top pulse shape can be produced by inserting anoptional amplitude filter 54 as shown in FIG. 15 in front of the lastamplifier module to produce a flat-top pulse spectrum. A flat-topspectrum is indeed transformed into a flat-top pulse after sufficientpulse stretching, because there is a direct relation between spectralcontent and time delay after sufficient pulse stretching. It can beshown that even values of self-phase modulation as large as 10*π can betolerated for flat-top pulses without incurring significant pulsedistortions.

An amplitude filter as shown in FIG. 15 may in turn also be used tocontrol the amount of higher-order dispersion in the amplifier chain forstrongly chirped pulses in the presence of self-phase modulation whenreshaping of the pulse spectrum in the amplifier can be neglected, i.e.,outside the regime where parabolic pulses are generated. In this caseself-phase modulation produces an effective amount of higher-orderdispersion of:

${\beta_{n}^{SPM} = {{{\gamma P}_{0}L_{eff}\frac{^{n}{S(\omega)}}{\; \omega^{n}}}_{\omega = 0}}},$

where P₀ is the peak power of the pulse and S(ω) is the normalized pulsespectrum. L_(eff) is the effective nonlinear lengthL_(eff)=[exp(gL)−1]/g, where L is the amplifier length and g is theamplifier gain per unit length. Thus by accurately controlling thespectrum of strongly chirped pulses with an amplitude filter as shown inFIG. 15, any amount of higher-order dispersion can be introduced tocompensate for the values of higher-order dispersion in a chirped pulseamplification system. It can indeed be shown for 500 fs pulses stretchedto around 1 ns, a phase shift of ≈10 π is sufficient to compensate forthe third-order dispersion of a bulk grating compressor (as shown inFIG. 5) consisting of bulk gratings with 1800 grooves/mm. Attractivewell-controllable amplitude filters are for example fiber transmissiongratings, though any amplitude filter may be used to control the pulsespectrum in front of such a higher-order dispersion inducing amplifier.

As another embodiment for the combination of an amplifier module with apulse picker, the configuration displayed in FIG. 16 can be used. Sincevery high energy pulses require large core multi-mode fibers for theiramplification, the control of the fundamental mode in a single-passpolarization maintaining fiber amplifier may be difficult to accomplish.In this case, it may be preferred to use a highly centro-symmetricnon-polarization maintaining amplifier to minimize mode-coupling and toobtain a high-quality output beam. To obtain a deterministicenvironmentally stable polarization output from such an amplifier, adouble-pass configuration as shown in FIG. 16 may be required. Here asingle-mode fiber 55 is used as a spatial mode filter after the firstpass through the amplifier 56; alternatively, an aperture could be usedhere. The spatial mode filter 55 cleans up the mode after the first passthrough the multi-mode amplifier 56, and also suppresses amplifiedspontaneous emission in higher-order modes that tends to limit theachievable gain in a multi-mode amplifier. Lenses 60 can be used forcoupling into and out of amplifier 56, spatial mode filter 55, and pulsepickers 52 a and 52 b. The Faraday rotator 57 ensures that the backwardpropagating light is polarized orthogonal to the forward propagatinglight; the backward propagating light is coupled out of the system atthe shown polarization beam splitter 58. To optimize the efficiency ofthe system, a near-diffraction limited source is coupled into thefundamental mode of the multi-mode fiber 56 at the input of the system,where gain-guiding can also be used to further improve the spatialquality of the beam amplified in the multi-mode fiber. To count-down therepetition rate of the pulse train delivered from a SM and to suppressamplified spontaneous emission in the multi-mode amplifier, a 1stoptical modulator 52 a can be inserted after the first pass through themulti-mode amplifier. An ideal location is just in front of thereflecting mirror 59 as shown. As a result a double-pass gain as largeas 60-70 dB could be obtained in such a configuration, minimizing thenumber of amplification stages required from amplifying seed pulses withpJ energies up to the mJ energy level. This type of amplifier is fullycompatible with the SMs, PSMs and PCMs as discussed before, allowing forthe generation of femtosecond pulses with energies in the mJ regime. Asanother alternative for the construction of a high-gain amplifiermodule, a count-down of the repetition rate from a pulse train deliveredby a SM can also be performed with an additional 2nd modulator 52 bprior to injection into the present amplifier module as also shown inFIG. 16. The repetition rate of transmission windows of the 1stmodulator 52 a should then be either lower or equal to the repetitionrate of the transmission window of the 2nd modulator 52 b. Such aconfiguration is not separately shown. FIG. 16 shares some similaritieswith FIG. 5 of U.S. Pat. No. 5,400,350, which is hereby incorporated byreference.

FIG. 17 represents an embodiment of the femtosecond fiber oscillatorembodied in a fiber laser cavity 100. A polarization-maintaining gainfiber 101 has a core 102 and cladding region 103. The fiber core 102 isdoped with rare-earth ions, such as Yb, Nd, Er, Er/Yb, Tm or Pr, toproduce gain at a signal wavelength when the laser is pumped with diodelaser 104. The fiber core can be single-mode or multi-mode. The fiberlaser cavity 100 further contains an integrated fiber polarizer 105 anda chirped fiber Bragg grating 106. Both of these elements, 105 and 106,are generally constructed of short fiber pigtails (e.g., 0.001-1 m inlength), which are preferably fusion-spliced to fiber 101 using splices107, 108 and 109. Alternatively, fiber polarizer 105 can be spliced infront of beam expander 110. When using multi-mode fiber, splice 107 isselected to match the fundamental mode in the gain fiber 101.

An exemplary integrated fiber polarizer in accordance with the inventioncomprises a polarization-maintaining undoped polarizer fiber (PF), withtwo orthogonal polarization axes, where the loss along one polarizationaxis is significantly higher than the loss along the other polarizationaxis. Alternatively, a very short section (less than 1 cm) ofnon-birefringent fiber (i.e., non-polarization-maintaining fiber) can besandwiched between two sections of polarization-maintaining fiber, wherethe polarization axes of the polarization-maintaining fibers are alignedwith respect to each other. By side-polishing the non-birefringentfiber, e.g., down to the evanescent field of the fiber core, along oneof the axes of the birefringent fiber, and coating the polished regionwith metal, high extinction polarization action can be obtained alongone of the axes of the birefringent fiber. The design of side-polishedfiber polarizers is well known in the field and not discussed furtherhere.

For optimum laser operation, the fiber polarization axes of the PF arealigned parallel to the polarization axes of the gain fiber 101. Toensure stable modelocked operation, the polarizer preferably effectivelyeliminates satellite pulses generated by any misalignment between thepolarization axes of the PF and the gain fiber 101.

Neglecting any depolarization in the all-fiber polarizer itself, it canbe shown by applying a Jones matrix calculation method that for amisalignment of the polarization axes of gain fiber 101 and fiberpolarizer 105 by a degrees, the linear reflectivity R from theright-hand side of the cavity varies approximately between R=1−0.5 sin²2α and R=1 depending on the linear phase in the gain fiber 101. If thegroup delay along the two polarization axes of the gain fiber is largerthan the intra-cavity pulse width, any satellite pulse is suppressed bysin⁴α after transmission through the polarizer. Typical fiber splicingmachines can align polarization-maintaining fibers with an angularaccuracy of less than ±2°; hence any reflectivity variation due todrifts in the linear phase between the two polarization eigenmodes offiber 101 can be kept down to less than 3×10⁻³, whereas (forsufficiently long fibers) any satellite pulses obtained aftertransmission through the polarizer can be kept down to less than 6×10⁻⁶when using an integrated polarizer.

The chirped fiber Bragg grating 106 is preferably spliced to the PF 105at splice position 108 and written in non-polarization-maintainingfiber. In order to avoid depolarization in the fiber Bragg grating, theBragg grating pig-tails are preferably kept very short, e.g., a lengthsmaller than 2.5 cm is preferable between splice locations 108 and 109.To obtain a linear polarization output, a polarization-maintaining fiberpig-tail is spliced to the left-side of the fiber Bragg grating atsplice location 109. The laser output is obtained at a first fiber (orcavity) end 111, which is preferably angle-cleaved to avoidback-reflections into the cavity. An alternative preferred design iswith the fiber grating written in polarization-maintaining fiber.

Fiber Bragg grating 106 serves two functions. First, it is used as anoutput mirror (i.e., it feeds part of the signal back to the cavity)and, second, it controls the amount of cavity dispersion. In the presentimplementation, the chirped fiber Bragg grating has a negative(soliton-supporting) dispersion at the emission wavelength in thewavelength region near 1060 nm and it counter-balances the positivematerial dispersion of the intra-cavity fiber. To produce the shortestpossible pulses (with an optical bandwidth comparable to or larger thanthe bandwidth of the gain medium), the absolute value of the gratingdispersion is selected to be within the range of 0.5-10 times theabsolute value of the intra-cavity fiber dispersion. Moreover, the fiberBragg grating is apodized in order to minimize any ripple in thereflection spectrum of the grating. Accordingly, the oscillation ofchirped pulses is enabled in the cavity, minimizing the nonlinearity ofthe cavity and maximizing the pulse energy. Chirped pulses arecharacterized in having a pulse width which is longer than the pulsewidth that corresponds to the bandwidth limit of the corresponding pulsespectrum. For example the pulse width can be 50%, 100%, 200% or morethan 1000% longer than the bandwidth limit.

Alternatively, the oscillation of chirped pulses is also enabled byusing negative dispersion fiber in conjunction with positive dispersionchirped fiber Bragg gratings. Pulses with optical bandwidth comparableto the bandwidth of the gain medium can also be obtained with thisalternative design.

A SAM 112 at a second distal fiber end 113 completes the cavity. In anexemplary implementation a thermally expanded core (TEC) 110 isimplemented at cavity end 113 to optimize the modelocking performanceand to allow close coupling of the SAM 112 to the second fiber end 113with large longitudinal alignment tolerances. Etalon formation betweenthe fiber end 113 and the SAM 112 is prevented by an anti-reflectioncoating deposited on fiber end 113 (not separately shown). In thevicinity of the second fiber end 113, fiber 101 is further inserted intoferrule 114 and brought into close contact with SAM 112. Fiber 101 issubsequently fixed to ferrule 114 using, for example, epoxy and theferrule itself is also glued to the SAM 112.

The pump laser 104 is coupled into the gain fiber 101 via a lens systemcomprising, for example, two lenses 115 and 116 and a V-groove 117 cutinto fiber 101. Such side-coupling arrangements are described in, forexample, U.S. Pat. No. 5,854,865 ('865) to L. Goldberg et al.Alternatively, fiber couplers can be used for pump light coupling.

An exemplary design for a SAM in accordance with the present inventionis shown in FIG. 18 a. For example, SAM 200 includes an InGaAsP layer201 with a thickness of 50-2000 nm. Further, layer 201 is grown with aband edge in the 1 μm wavelength region; the exact wavelength is definedby the sought emission wavelength of the fiber laser and can varybetween 1.0-1.6 μm. The InGaAsP layer 201 is further coated or processedwith a reflective material such as Au or Ag. A dielectric mirror orsemiconductor Bragg reflector 202 is located beneath layer 201 and theentire structure is attached to heat sink 203, based on, for example,metal, diamond or sapphire.

In order to cover a broad spectral range (e.g., greater than 100 nm)metallic mirrors are preferred. When using a metallic mirror it isadvantageous to remove the substrate (InP) by means of etching. Whenusing HCl as an etching solvent the etching selectivity between InGaAsPand InP can be low, depending on the compound composition of InGaAsP. Anetch-stop layer is beneficial between the substrate and the InGaAsPlayer. InGaAs can be a proper etch-stop layer. When adding an InGaAslayer with a band-gap wavelength shorter than 1.03 μm, latticerelaxations can be avoided by keeping the thickness below 10 nm.

The InGaAsP layer can further be anti-reflection coated with layer 204on its upper surface to optimize the performance of the SAM. Because ofthe saturable absorption by InGaAsP, the reflectivity of the SAMincreases as a function of light intensity, which in turn favors thegrowth of short pulses inside the laser cavity. The absence of Al in thesaturable absorber layer prevents oxidization of the semiconductorsurfaces in ambient air and thus maximizes the life-time and powerhandling capability of the structure.

Instead of InGaAsP, any other Al-free saturable semiconductor can alsobe used in the construction of the SAM. Alternatively, Al-containingsemiconductors can be used in the SAM with appropriately passivatedsurface areas. Surface passivation can, for example, be accomplished bysulfidization of the semiconductor surface, encapsulating it with anappropriate dielectric or with an Al-free semiconductor cap layer. AnAlGaInAs absorber layer grown lattice-matched on InP can besurface-passivated with a thin (about 10 nm range) cap layer of InP.AlGaInAs with a higher band gap energy than the absorber layer can alsobe used for a semiconductor Bragg reflector in combination with InP.Among concepts for semiconductor Bragg mirrors lattice-matched to InP,an AlGaInAs/InP combination has an advantage over an InGaAsP/InP Braggreflector due to its high refractive index contrast.

Instead of a bulk semiconductor saturable absorber, a MQW saturableabsorber structure as shown in FIG. 18 b may also be used. In this case,the SAM 205 conveniently comprises MQW structures 206, 207 and 208separated by passive spacer layers 209-212 in order to increase thesaturation fluence and depth-selective ion-implantation concentration ofeach MQW section. Additional MQW structures can further be used,similarly separated by additional passive spacer layers. To reduce thewavelength and location sensitivity of the MQW saturable absorbers, thewidth of the spacer layers varies from spacer layer to spacer layer.Furthermore, multiple bulk layers with thicknesses larger than 500 Å canreplace the MQW structure. The MQW layers, in turn, can contain severallayers of quantum wells and barriers such as, for example, InGaAs andGaAs, respectively. Top surface 209 can further be anti-reflectioncoated (not shown); a reflective structure is obtained by includingmirror structure 213. The entire structure can be mounted on heat sink214.

The control of the response time of the saturable absorption forconcomitant existence of fast and slow time constants is realized byintroducing carrier trap centers with depth controlled H+ (or otherions) implantation. The implantation energy and dose are adjusted suchthat part of the absorbing semiconductor film contains a minimal numberof trap centers. For example the semiconductor layer with the minimalnumber of trap centers can be selected to be at the edge of the opticalpenetration range of exciting laser radiation. Such a design serves onlyas an example and alternatively any semiconductor area within theoptical penetration range can be selected to contain a minimal number oftrap centers. Hence distinctive bi-temporal carrier relaxation isobtained in the presence of optical excitation. As an illustration ofdepth selective ion implantation, FIG. 19 shows the measurement of thedepth profile of H+ ion implantation of an InGaAsP absorber film takenfrom secondary ion mass spectroscopy (SIMS).

The obtained bi-temporal carrier life-time obtained with thesemiconductor film with a proton concentration as shown in FIG. 19, isfurther illustrated in FIG. 20. Here the reflectivity modulation (dR/R0)of a semiconductor saturable mirror due to excitation of the saturablemirror with a high energy short pulse at time t=0 is shown as a functionof time delay. The measurement was obtained with a pump-probe technique,as well known in the art. FIG. 20 clearly displays the bi-temporalresponse time due to fast (<1 ps) and slow (>>100 ps) recovery. Thedistinctive fast response originates from the depth region with hightrap concentration, while the slow component results from the rear depthregion with a much lower trap center concentration.

When employing this absorber in the laser system described with respectto FIG. 17, Q-switched mode-locking is obtained at intracavity powerlevels of a few mW. At the operating pump power level, stable cwmode-locking evolving from Q-switch mode-locking is observed. Incontrast, no Q-switching and no mode-locking operation is obtained withthe same semiconductor material implanted uniformly with protons withoutbi-temporal carrier relaxation (exhibiting only fast carrierrelaxation).

We emphasize that the description for FIG. 19 and FIG. 20 is to serve asan example in controlling 1) the fast time constant, 2) the slow timeconstant, 3) the ratio of the fast and slow time constants, 4) theamplitude of the fast response, 5) the amplitude of the slow response,and finally 6) the combination of all of the above by ion implantationin a saturable absorber. Thus, the concept depicted hereby can beapplicable for any type of laser modelocked with a saturable absorber.Specifically, in the presence of un-avoidable large spuriousintra-cavity reflections such as in fiber lasers or thin disk lasers (F.Brunner et al., Sub-50 fs pulses with 24 W average power from apassively modelocked thin disk Yb:YAG laser with nonlinear fibercompression, Conf. on Advanced Solid State Photonics, ASSP, 2003, paperNo.: TuA1), the disclosed engineerable bi-temporal saturable absorberscan greatly simplify and stabilize short pulse formation.

The preferred implantation parameters for H+ ions in GaAs or InP relatedmaterials including MQW absorbers are as follows: The doses and theimplantation energies can be selected from 10¹² cm⁻² to 10¹⁷ cm⁻² andfrom 5 keV to 200 keV, respectively, for an optically absorbing layerthickness between 50 nm and 2000 nm. For MQW absorbers, the selectiveion-implantation depth is rather difficult to measure because theshallow MQW falls into the implantation peak in FIG. 19. However, withthe separation of MQW sections with spacers 209-212 (as shown in FIG.18) it is feasible to employ depth selective ion implantation. Forarsenic implantation, the implantation parameters for 50-2000 nmabsorbing layer spans from 10¹² cm⁻² to 10¹⁷ cm⁻² for the dosage and animplantation energy range of 100 keV to 1000 keV. In case of MQWsaturable absorbers, the implantation range is preferably selectedwithin the total thickness of the semiconductor layer structurecontaining MQW sections and spacers. In addition to H⁺ and arsenic, anyother ions such as for example Be can be implanted with controlledpenetration depth by adjusting the above recipes according to thestability requirements of the desired laser.

FIG. 21 a illustrates an alternative implementation of the fiber end andSAM coupling in FIG. 17. Here cavity 300 comprises an angle-polishedthermal-diffusion expanded core (TEC) 301. Fiber end 302 is brought intoclose contact with SAM 303 and fiber 304 is rotated inside ferrule 305to maximize the back reflection from SAM 303. Ferrule 305 is furtherangle-polished and SAM 303 is attached to the angle-polished surface offerrule 305. As shown in FIG. 21 a, fiber 304 is conveniently glued tothe left-hand side of ferrule 305. A wedge-shaped area between the fibersurface 302 and SAM 303 greatly reduces the finesse of the etalonbetween the two surfaces, which is required for optimum modelocked laseroperation.

Instead of TEC cores, more conventional lenses or graded index lensescan be incorporated between the fiber end and the SAM to optimize thebeam diameter on the SAM. Generally, two lenses are required. A firstlens collimates the beam emerging from the fiber end, and a second lensfocuses the beam onto the SAM. According to present technology, evenconventional lenses allow the construction of a very compact package forthe second fiber end. An implementation with two separate collimationand focusing lenses is not separately shown. To minimize unwanted backreflections into the fiber cavity and to minimize the number ofcomponents, a single lens can be directly fused to the fiber end asdepicted in FIG. 21 b. As shown in FIG. 21 b, assembly 306 contains SAM303 and fiber 304 as well as lens 307, which focuses the optical beamonto the SAM. Lens 307 can also include a graded index lens.

To minimize aberrations in assembly 306, an additional lens can also beincorporated between lens 307 and SAM 303. Such an assembly is notseparately shown. Alternatively, a lens can be directly polished ontofiber 304; however, such an arrangement has the disadvantage that itonly allows a beam size on the SAM which is smaller than the beam sizeinside the optical fiber, thereby somewhat restricting the designparameters of the laser. To circumvent this problem, a lens surface canbe directly polished onto the surface of a TEC; such an implementationis not separately shown. Another alternative is to exploit agraded-index lens design attached directly onto the fiber tip to varythe beam size on the SAM. In the presence of air-gaps inside theoscillator a bandpass filter 308 can be incorporated into the cavity,allowing for wavelength tuning by angular rotation as shown, forexample, in FIG. 21 b.

Passive modelocking of laser cavity 100 (FIG. 17) is obtained when thepump power exceeds a certain threshold power. In a specific, exemplary,implementation, polarization-maintaining fiber 101 was doped with Ybwith a doping level of 2 weight %; the doped fiber had a length of 1.0m; the core diameter was 8 um and the cladding diameter was 125 um. Anadditional 1.0 m length of undoped polarization-maintaining fiber wasalso present in the cavity. The overall (summed) dispersion of the twointra-cavity fibers was approximately +0.09 ps². In contrast, the fibergrating 106 had a dispersion of −0.5 ps², a spectral bandwidth of 10 nmand a reflectivity of 50%. The grating was manufactured with a phasemask with a chirp rate of 80 nm/cm.

When pumping with an optical power of 1.0 W at a wavelength of 910 nm,the laser produced short chirped optical pulses with a full width halfmaximum width of 1.5 ps at a repetition rate of 50 MHz. The averageoutput power was as high as 10 mW. The pulse bandwidth was around 2 nmand hence the pulses were more than two times longer than thebandwidth-limit which corresponds to around 800 fs.

Alternatively, a fiber grating 106 with a dispersion of −0.1 ps²,closely matching the dispersion of the intra-cavity fiber, wasimplemented. The fiber grating had a reflectivity of 9% and a spectralbandwidth of 22 nm centered at 1050 nm. The grating was manufacturedwith a phase mask with a chirp rate of 320 nm/cm. The laser thenproduced chirped optical pulses with a full-width half maximum width of1.0 ps at a repetition rate of 50 MHz with an average power of 25 mW.The pulse spectral bandwidth was around 20 nm and thus the pulses werearound 10 times longer than the bandwidth limit, which corresponds toaround 100 fs. The generation of pulses with a pulse width correspondingto the bandwidth limit was enabled by the insertion of a pulsecompressing element; such elements are well known in the state of theart and are not further discussed here. The generation of even shorterpulses can be generated with fiber gratings with a bandwidth of 40 nm(and more) corresponding to (or exceeding) the spectral gain bandwidthof Yb fibers.

Shorter pulses or pulses with a larger bandwidth can be convenientlyobtained by coupling the fiber output into another length of nonlinearfiber as shown in FIG. 22. Here, assembly 400 contains the integratedfiber laser 401 with pig-tail 402. Pig-tail 402 is spliced (orconnected) to the nonlinear fiber 403 via fiber splice (or connector)404. Any type of nonlinear fiber can be implemented. Moreover, fiber 403can also comprise a fiber amplifier to further increase the overalloutput power.

In addition to cladding pumped fiber lasers, core-pumped fiber laserscan be constructed in an integrated fashion. Such an assembly is shownin FIG. 23. The construction of cavity 500 is very similar to the cavityshown in FIG. 17. Cavity 500 contains polarization-maintaining fiber 501and integrated fiber polarizer 502. Fiber 501 is preferably single-clad,though double-clad fiber can also be implemented. The chirped fibergrating 503 again controls the dispersion inside the cavity and is alsoused as the output coupler. Fiber 501, fiber polarizer 502, fibergrating 503 and the polarization-maintaining output fiber are connectedvia splices 504-506. The output from the cavity is extracted atangle-cleaved fiber end 507. SAM 508 contains anti-reflection coatedfiber end 509, located at the output of the TEC 510. Fiber 501 and SAM508 are fixed to each other using ferrule 511. The fiber laser is pumpedwith pump laser 512, which is injected into the fiber viawavelength-division multiplexing coupler 513.

In addition to chirped fiber gratings, unchirped fiber gratings can alsobe used as output couplers. Such cavity designs are particularlyinteresting for the construction of compact Er fiber lasers. Cavitydesigns as discussed with respect to FIGS. 17 and 23 can be implementedand are not separately shown. In the presence of fiber gratings as shownin FIGS. 17 and 23, the fiber gratings can also be used as wavelengthtuning elements. In this, the fiber gratings can be heated, compressedor stretched to change their resonance condition, leading to a change incenter wavelength of the laser output. Techniques for heating,compressing and stretching the fiber gratings are well known.Accordingly, separate cavity implementations for wavelength tuning via amanipulation of the fiber grating resonance wavelength are notseparately shown.

In the absence of a fiber grating, a mirror can be deposited or attachedto one end of the fiber cavity. The corresponding cavity design 600 isshown in FIG. 24. Here, it is assumed that the fiber 601 is core pumped.The cavity comprises an intra-cavity all-fiber polarizer 602 spliced tofiber 601 via splice 603. Another splice 604 is used to couple WDM 605to polarizer 602. Polarization maintaining WDM 605 is connected to pumplaser 606, which is used to pump the fiber laser assembly. Saturableabsorber mirror assembly 607, as described previously with respect toFIGS. 17 and 23, terminates one cavity end and is also used as thepassive modelocking element.

A second fiber polarizer 608 is spliced between WDM 605 andpolarization-maintaining output coupler 609 to minimize the formation ofsatellite pulses, which can occur when splicing sections of polarizationmaintaining fiber together without perfect alignment of their respectivepolarization axes, as discussed in U.S. patent application Ser. No.09/809,248. Typically, coupler 609 has a coupling ratio of 90/10 to50/50, i.e., coupler 609 couples about 90-50% of the intra-cavity signalout to fiber pig-tail 610. Pig-tail 610 can be spliced to a fiberisolator or additional fiber amplifiers to increase the pulse power. Thesecond cavity end is terminated by mirror 611. Mirror 611 can bedirectly coated onto the fiber end face or, alternatively, mirror 611can be butt-coupled to the adjacent fiber end.

The increase in stability of cavity 600 compared to a cavity where theoutput coupler fiber, the WDM fiber and gain fiber 601 are directlyconcatenated without intra-fiber polarizing stages, can be calculatedusing a Jones matrix formalism even when coherent interaction betweenthe polarization axes of each fiber section occurs.

Briefly, due to the environmental sensitivity of the phase delay betweenthe polarization eigenmodes of each fiber section, for N directlyconcatenated polarization-maintaining fibers the reflectivity of a fiberFabry-Perot cavity can vary between R=1 and R=1−(N×α)², where α is theangular misalignment between each fiber section. Further, it is assumedthat α is small (i.e., α<<10°) and identical between each pair of fibersections. Also, any cavity losses are neglected. In fact, it isadvantageous to analyze the possible leakage L into the unwantedpolarization state at the output of the fiber cavity. L is simply givenby L=1−R. For the case of N concatenated fiber sections, the maximumleakage is thus (N×α)².

In contrast, a cavity containing N−1 polarizers in-between N sections ofpolarization-maintaining fiber is more stable, and the maximum leakageis L=2×(N−1)α². Here, any depolarization in the fiber polarizers itselfis neglected. For instance, in a case where N=3, as in cavity 600, theleakage L into the wrong polarization axis is 2×(3−1)/3³=4/9 timessmaller compared to a cavity with three directly concatenated fibersections. This increase in stability is very important in manufacturingyield as well as in more reproducible modelocked operation in general.

In constructing a stable laser, it is also important to consider theconstruction of WDM 605 as well as output coupler 609. Various vendorsoffer different implementations. An adequate optical representation ofsuch general polarization-maintaining fiber elements is shown in FIG.25. It is sufficient to assume that a general coupler 700 comprises twopolarization-maintaining fiber sections (pig-tails) 701, 702 with acoupling point 703 in the middle, where the two polarization axes of thefiber are approximately aligned with respect to each other.

In order to ensure pulse stability inside a passively modelocked laser,the group-velocity walk-off along the two polarization axes of fibersections 701, 702 should then be longer than the full-width half maximum(FWHM) pulse width of the pulses generated in the cavity. For example,assuming a birefringent fiber operating at a wavelength of 1550 nm witha birefringence of 3×10⁻⁴ corresponding to a polarization beat length of5 mm at 1550 nm, the stable oscillation of soliton pulses with a FWHMwidth of 300 fs requires pig-tails with a length greater than 29 cm. For500 fs pulses, the pig-tail length should be increased to around 50 cm.

Referring back to FIG. 24, if a fiber pig-tailed output is not required,mirror 611 as well as output coupler 609 can be omitted, and the 4%reflection from the fiber end adjacent to mirror 611 can be used as aneffective output mirror. Such an implementation is not separately shown.

Alternatively, a fiber-pig-tail can be butt-coupled to mirror 611 andalso be used as an output fiber pigtail. Such an implementation is shownin FIG. 26. Here, cavity 800 comprises core-pumped fiber 801, fiberpolarizer 802 and SAM assembly 803. The laser is pumped via WDM 804connected to pump laser 805. An appropriate mirror (or mirror coating)806 is attached to one end of the cavity to reflect a part of theintra-cavity light back to the cavity and to also serve as an outputmirror element. Fiber pig-tail 807 is butt-coupled to the fiber laseroutput mirror 806 and an additional ferrule 808 can be used to stabilizethe whole assembly. The polarization axes of fiber 807 and 801 can bealigned to provide a linearly polarized output polarization. Again,applying a Jones matrix analysis, cavity 800 is more stable than cavity600, because it comprises only one intra-fiber polarizing section. Themaximum leakage in cavity 800 compared to a cavity comprising directlyconcatenated WDM and gain fiber sections is 50% smaller.

Similarly, a cladding pumped version of cavity 600 can be constructed.Cavity 900 shown in FIG. 27 displays such a cavity design. Fiber 901 ispumped via pump laser 902, which is coupled to fiber 901 via lensassembly 903 and 904 as well as V-groove 905. Alternatively,polarization-maintaining multi-mode fiber couplers or star-couplerscould be used for pump power coupling. Such implementations are notseparately shown. One end of the laser cavity is terminated with SAMassembly 906 (as discussed in regard to FIGS. 17, 23 and 24, which isalso used as the modelocking element. A single-polarization inside thelaser is selected via all-fiber polarizer 907, which is spliced into thecavity via splices 908 and 909. Polarization-maintaining output coupler910 is used for output coupling. The laser output is extracted via fiberend 911, which can further be spliced to additional amplifiers. Cavitymirror 912 terminates the second cavity end. Output coupler 910 canfurther be omitted and the laser output can be obtained via abutt-coupled fiber pig-tail as explained with reference to FIG. 30.

The cavity designs discussed with respect to FIGS. 17, 23, 24, 26 and 27follow general design principles as explained with reference to FIGS. 28a-28 c.

FIG. 28 a shows a representative modelocked Fabry-Perot fiber lasercavity 1000, producing a linear polarization state oscillating insidethe cavity containing one (or more) sections of non-polarizationmaintaining fiber 1001 and one (or more) sections of polarizationmaintaining fiber 1002, where the length of fiber section 1001 issufficiently short so as not to degrade the linear polarization stateinside the fiber laser cavity, more generally a predominantly linearpolarization state is oscillating everywhere within the intracavityfiber. The fiber laser output can be obtained from cavity end mirrors1003 or 1004 on either side of the cavity. To suppress the oscillationof one over the other linear polarization state inside the cavity,either fiber 1001 or 1002 has a polarization dependent loss at theemission wavelength.

FIG. 28 b shows a representative modelocked Fabry-Perot fiber lasercavity 1005, producing a linear polarization state oscillating insidethe cavity containing two (or more) sections of polarization maintainingfibers 1006, 1007, where the length of fiber sections 1006, 1007 issufficiently long so as to prevent coherent interaction of short opticalpulses oscillating inside the cavity and propagating along thebirefringent axes of fibers 1006, 1007. Specifically, for an oscillatingpulse with a FWHM width of τ, the group delay of the oscillating pulsesalong the two polarization axes of each fiber should be larger than τ.For oscillating chirped pulses τ represents the bandwidth-limited pulsewidth that corresponds to the oscillating pulse spectrum. Cavity 1005also contains end mirrors 1008 and 1009 and can further containsufficiently short sections of non-polarization maintaining fiber asdiscussed with reference to FIG. 28 a.

FIG. 28 c shows a representative modelocked Fabry-Perot fiber lasercavity 1010, producing a linear polarization state oscillating insidethe cavity containing one (or more) sections of polarization maintainingfiber 1011, 1012 and one (or more) sections of polarizing fiber (orall-fiber polarizer) 1013, where the length of fiber sections 1011, 1013is not sufficient to prevent coherent interaction of short opticalpulses oscillating inside the cavity and propagating along thebirefringent axes of fibers 1011, 1013, where the polarizing fiber issandwiched between the sections of short polarization maintaining fiber.Cavity 1010 further contains cavity end mirror 1014 and 1015 and canfurther contain short sections of non-polarization maintaining fiber asdiscussed with reference to FIG. 28 a. Moreover, cavity 1010 (as well as1000 and 1005) can contain bulk optic elements 1016, 1017 (or any largernumber) randomly positioned inside the cavity to provide additionalpulse control such as wavelength tuning or dispersion compensation. Notethat the fibers discussed here can be single-clad, double-clad; thefibers can comprise also holey fibers or multi-mode fibers according tothe system requirement. For example polarization maintaining holeyfibers can be used for dispersion compensation, whereas multi-modefibers can be used for maximizing the output pulse energy. Cavitymirrors 1014, 1015, 1003, 1004 and 1008, 1009 can further comprise bulkmirrors, bulk gratings or fiber gratings, where the fiber gratings canbe written in short sections of non-polarization maintaining fiber thatis short enough so as not to perturb the linear polarization stateoscillating inside the cavity.

FIG. 29 serves as an example of a passively modelocked linearpolarization cavity containing holey fiber for dispersion compensation.Cavity 1100 contains fiber 1101, side-pumping assembly 1102 (directingthe pump light either into the cladding or the core of fiber 1101 asexplained before), saturable absorber mirror assembly 1103, all fiberpolarizer 1104 and fiber output coupler 1105 providing an output atfiber end 1106. All the above components were already discussed. Inaddition, a length of polarization maintaining holey fiber 1006 isspliced to the cavity for dispersion compensation and the cavity isterminated on the left hand side by mirror 1107.

FIG. 30 serves as another example of a passively modelocked linearpolarization cavity containing a fiber grating for dispersioncompensation as applied to the generation of ultra-stable spectralcontinua. System 1400 comprises a small modification of the cavityexplained with respect to FIG. 23. System 1400 contains a fiber laser1401 generating pulses with a bandwidth comparable to the spectralbandwidth of the fiber gain medium 1402. Fiber laser 1401 furthercomprises saturable absorber mirror assembly 1403, wide bandwidth fibergrating 1404, polarization maintaining wavelength division multiplexing(WDM) coupler 1405, which is used to direct pump laser 1406 into fibergain medium 1402. Pump laser 1406 is preferably single-mode to generatethe least amount of noise.

To enable the oscillation of short pulses with a bandwidth comparable tothe bandwidth of the gain medium 1402, saturable absorber mirror 1403contains a bi-temporal saturable absorber, constructed with abi-temporal life-time comprising a 1^(st) short life-time of <5 ps and a2^(nd) long life-time of >50 ps. More preferable is a first life-time of<1 ps, to allow pulse shaping of pulses as short as 100 fs and shorter.By selecting the penetration depth of the implanted ions into thesaturable absorber, even tri-temporal saturable absorbers can beconstructed.

The wide-bandwidth grating is preferably selected to approximately matchthe dispersion of the intra-cavity fibers. The wide-bandwidth gratingcan be made in short non-polarization maintaining fibers and it can bemade also in polarization maintaining fibers. In order to suppressdetrimental effects from cross coupling between the two polarizationaxes of the fiber grating, coupling to cladding modes in such largebandwidth fiber gratings should be suppressed. Gratings with suppressedcoupling to cladding modes can be made in optical fibers withphotosensitive core and cladding area, where the photosensitive claddingarea is index-matched to the rest of the cladding. Such fiber designsare well known in the state of the art and can for example bemanufactured with an appropriate selection of germania and fluorinedoping in the core and cladding regions and such fiber designs are notfurther discussed here. Because of the large generated bandwidth,splicing of such polarization maintaining gratings to the rest of thecavity without coherent coupling between the linear polarizationeigenmodes is no problem. Alternatively, the fiber gratings can bewritten directly into the photosensitive gain fiber, with an index anddopant profile that suppresses coupling to cladding modes in the fibergrating.

To sustain large spectral bandwidth, fiber grating 1404 has preferably aspectral bandwidth>20 nm. A splice 1407 (or an equivalent bulk opticlens assembly) is used to connect the output of fiber laser 1401 tononlinear fiber 1408 to be used for additional spectral broadening ofthe output of the fiber laser. For example fiber 1408 can comprise ahighly nonlinear dispersion-flattened holy fiber. In conjunction withsuch fiber, smooth broad-bandwidth spectral profiles with bandwidthsexceeding 100 nm can be generated. These spectral outputs can be useddirectly in high precision optical coherence tomography.

The pulses at the output of fiber 1408 are generally chirped and adispersion compensation module 1409 can be inserted after the outputfrom fiber 1408 for additional pulse compression. The dispersioncompensation module can be spliced directly to fiber end 1408 whenoptical fiber is used for dispersion compensation. Alternatively, thedispersion compensation module can comprise two (or one) bulk grating(or prism) pair(s). Such bulk optic elements for dispersion compensationare well known in the state of the art and are not further discussedhere. Coupling into and out of a bulk dispersion compensating module isobtained via lenses 1410 and 1411. The output can also be from the otherend of the cavity. The pulses generated after pulse compression can beas short as 20-200 fs. As mentioned previously this pulse compressionmodule is optional and the dispersion compensation needed for thisoscillator can be compensated by the pulse stretcher before and pulsecompressor after the regenerative amplifier.

A fiber amplifier 1412 can also be added if further pulse energy isnecessary.

Note that the discussion with respect to FIG. 30 serves only as anexample of the use of bi- or multi-temporal saturable absorbers in thegeneration of mass producible ultra-broad band, low noise spectralsources. Other modifications are obvious to anyone skilled in the art.These modifications can comprise for example the construction of anintegrated all-fiber assembly substituting elements 1408, 1409-1411 and1412.

Though the discussion of the laser system with respect to FIG. 30 wasbased on the use of polarization maintaining fiber, non polarizationmaintaining fiber can also be used to produce pulses with bandwidthcomparable to the bandwidth of the gain medium. In this case, saturableabsorbers with depth controlled ion implantation are also of greatvalue. Essentially, any of the prior art modelocked fiber laser systemsdescribed above (that were using saturable absorbers) can be improvedwith engineered bi- and multi-temporal saturable absorbers.Specifically, any of the cavity designs described in U.S. Pat. Nos.5,450,427 and 5,627,848 to Fermann et al. can be used for the generationof ultra broadband optical pulses in conjunction with bi- ormulti-temporal saturable absorbers and wide-bandwidth fiber Bragggratings.

An embodiment with the fewest bulk optic components in the optical pathis shown in FIG. 31. The source of ultrashort pulses is a fiber-basedMOPA 100. This source is described in detail in Ser. No. 10/814,502which is incorporated herein. A polarization-maintaining gain fiber 101has a core 102 and cladding region 103. The fiber core 102 is doped withrare-earth ions, preferably Yb, to produce gain at a signal wavelengthwhen the laser is pumped with diode laser 104. The pump diode is coupledinto the cladding region 103 of fiber 101 using for example two lenses105 and 106 and V-groove 107, though coupling systems comprising morethan two lenses can be used. Alternatively a WDM and a single mode laserdiode can be used for in core optical pumping. The fiber core can besingle-mode or multi-mode. The multi-mode fiber is designed to propagatesingle mode as is described in U.S. application Ser. No. 09/785,944(incorporated by reference herein). The multi-mode fiber can also bemulti-mode photonic crystal fiber as is described in Ser. No. 10/844,943(incorporated herein). The fiber laser cavity 100 further contains afiber Bragg grating 108, written in polarization maintaining fiber, anoptional polarizer (fiber or bulk) 109 and a saturable absorber assembly110. A bulk polarizer such as a cube polarizer is preferred. Fibergrating 108 can be chirped or un-chirped, where the polarization crosstalk between the two polarization axes of the polarization maintainingfiber containing the fiber gratings is preferably less than 15 dB. Fiberend face 111 completes the basic MOPA system. The fiber Bragg gratingcan be written directly into fiber 101 or it can be spliced into theMOPA system at splice positions 112 and 113, where the polarization axesof all involved fibers are aligned with respect to each other. The MOPAcomprises an oscillator assembly 114 and an amplifier assembly 115. Theoscillator assembly 114 is bounded on the left hand side by fibergrating 108 and on the right hand side by saturable absorber assembly110. The amplifier assembly 115 is bounded by fiber grating 108 andfiber end 111 on the two opposite distal ends. In the present examplefiber 101 is used both in the amplifier section and in the amplifiersection. In general, however, different fibers can be used in theoscillator and amplifier, though to avoid feedback from the amplifierinto the oscillator, the refractive index of both oscillator andamplifier fiber should be closely matched. The chirp of the outputpulses can be conveniently compensated with the delivery fiber 118,where lenses 116 and 117 are used to couple the output from the MOPAinto the delivery fiber. Other pulse modification elements can be placedbetween the lenses such as an isolator, tunable filter or fibergratings. The delivery fiber can comprise standard silica step-indexfiber, holey fiber or photonic crystal fiber. The use of photoniccrystal for dispersion compensation and pulse delivery was previouslydisclosed in Ser. No. 10/608,233. The delivery fiber 118 can also bespliced directly to fiber end face 111, enabling a further integrationof the laser assembly. The delivery fiber can also be sufficiently longto stretch the pulse sufficiently for amplification in the regenerativeamplifier. The need for a compressor depends on the exact design of theregenerative amplifier.

The embodiment in FIG. 31 may be the simplest design, however the pulseconditioning shown in FIG. 1 and described in Ser. No. 10/960,923 areoften necessary to obtain the needed specifications from the ultrafastsource. Ser. No. 10/814,319 (incorporated by reference herein) teacheshow to utilize various modules for pulse conditioning for a fiber lasersource. Ser. No. 10/813,163 (incorporated by reference herein) describesutilizing some of these methods in a fiber chirped pulse amplificationsystem. These pulse conditioning methods can be utilized in aregenerative amplifier system. FIG. 32 illustrates one embodiment of alaser system 550 having a monitoring and feedback control capability. Inone embodiment of the laser system, monitoring the performance such asoutput power at some point(s) of the system and providing feedback tothe diode pump drivers for active control can achieve a stableoperation. FIG. 10 illustrates one embodiment of a laser system 550having such a monitoring and feedback feature. The exemplary lasersystem 550 comprises an oscillator 552 coupled to an attenuator 556 viaan isolator 554. The output from the attenuator 556 is fed into abandpass filter 558 whose output is then directed to a stretcher 561 andthen an amplifier 560. The output from the amplifier 560 is fed into theregenerative amplifier 563 and then a compressor 564 via an isolator562. It should be noted that the use of the attenuator 556 and thebandpass filter 558 are exemplary, and that either of these componentsmay be excluded and any other modular components, including thosedisclosed herein, may be used in the laser system having feedback.

As shown in FIG. 32, the laser system 550 further comprises a firstmonitor component 570 that monitors a performance parameter of thesystem after the oscillator 552. The monitor 570 may comprise a sensorand controller. The monitor 570 may issue adjustment commands to a firstdriver 572 that implements those adjustment commands at the oscillator552.

The exemplary laser system 550 is shown to further comprise a secondmonitor component 574 that monitors a performance parameter of thesystem after the amplifier 560. The monitor 574 may similarly comprise asensor and controller. The monitor 574 can then issue adjustmentcommands to a second driver 576 that implements those adjustmentcommands at the amplifier 560.

The monitoring of the system performed by the exemplary monitors 570and/or 574 may comprise for example an optical detector and electronicsthat monitors optical intensity or power or other relevant parametersuch as, e.g., frequency and spectrum. In response to such measurement,the monitor and the driver may induce changes in the oscillator and/orthe amplifier by for example adjusting the pump intensity and/or rate,or adjusting the operating temperature. Temperature control of theoscillator can stabilize the gain dynamics as well as frequencyfluctuations. Temperature control of the amplifier can also be used tostabilize the gain dynamics.

Other configurations for providing feedback to control the operation ofthe laser system may also be employed. For example, more or lessfeedback loops may be included. The electronics associated with thesefeedback loops are further described in Ser. No. 10/813,173(incorporated by reference herein). A particularly important electroniccontrol is to control the gain of the fiber amplifier. At 1 KHzrepetition rate and lower, the gain of the fiber amplifier could bereduced between pulses to conserve the lifetime of the laser diode. Alsothe gain needs to be reduced on the fiber amplifier if a signal is lostfrom the short pulse source to protect from optical damage to the fiberamplifier or subsequent optical elements. The loops may involveelectronics that perform operations such as calculations to determinesuitable adjustments to be introduced. Examples are the mode-lockstart-up and search algorithms that are disclosed in Attorney Docket No.A8828 (incorporated by reference herein). The start-up algorithm isshown in FIG. 37. The feedback may be obtained from other locations inthe system and may be used to adjust other components as well. Theembodiments described in connection with FIG. 32 should not be construedto limit the possibilities.

A good Polarization Extinction Ratio (PER) is an important factor inmaintaining good temporal pulse quality in a fiber-based ultrafastsource for a regenerative amplifier. Poor polarization extinctioncreates ripple on the spectrum and on the chirped pulse. In variouspreferred embodiments, the light in the laser is linearly polarized. Thedegree of the linear polarization may be expressed by the polarizationextinction ratio (PER), which corresponds to a measure of the maximumintensity ratio between two orthogonal polarization component. Incertain embodiments, the polarization state of the source light may bemaintained by using polarization-maintaining single-mode fiber. Forexample, the pigtail of the individual modular device may be fabricatedwith a polarization-maintaining fiber pigtail. In such cases, the PER ofeach modular stage may be higher than about 23 dB. Ensuring a highpolarization extinction ratio throughout a series of modules ischallenging despite the use of single mode polarization maintainingfiber. Degradation of the PER can occur at the fiber ferrule, fiberholder, or fusion splice in the series of modules.

Levels of PER above 23 dB may be obtained in a system by utilizinglinear-polarizing optical components in the modules. Use oflinear-polarizing components in the modules within systems that containpolarization degrading elements such as a fiber ferrule, fiber holder,or fusion splice is advantageous. The linear polarizers counter thesuperposition of the phase shift from each polarization degradingelement. A superposed phase shift of 10 degrees may reduce the PER toabout 15 dB in which case intensity fluctuation through a linearpolarizer might be more than about 4%. In contrast, by embedding linearpolarizers throughout the series of modules, the PER of the aggregatesystem can be substantially controlled such that the intensityfluctuation is below about 1%, provided that the PER of the individualmodule and splice is above about 20 dB.

FIG. 33 a illustrates one embodiment of a module that can be utilizedfor polarization correction or as variable attenuation. It is a variableattenuator module 730 comprising a housing 732 that contains opticalcomponents for providing a controllable amount of optical attenuation.The housing 732 may be sealed and thermally insulated as well. A firstoptical fiber connector 734 comprising an optical fiber 736 having anangle polished or cleaved end face passes through one sidewall of thehousing 732 into an inner region of the housing containing the pluralityof optical components. These optical components include a first lens 738for collecting and preferably collimating light output from the opticalfiber 736, a variable wave plate 740 and a polarization selectiveoptical element 742. A second optical fiber connector 744 comprising anoptical fiber 745 having an angle polished or cleaved end face passesthrough another sidewall of the housing 732 into the inner regioncontaining the optical components. The variable waveplate 740 comprisesa rotatable waveplate mounted on a rotatable wheel 746 and thepolarization selective optical element 742 comprises a polarizationbeamsplitter such as a MacNeille prism. A second lens 748 disposedbetween the polarization selective optical element 742 couples lightbetween the polarization beamsplitter 742 and the second optical fiber745. An optical path is formed from the first optical fiber 736 throughthe waveplate 740 and prism 742 to the second optical fiber connector744.

The waveplate 740 can be rotated to vary the distribution of light intoorthogonal polarizations. The polarization beamsplitter 742 can be usedto direct a portion of the light out of the optical path between thefirst and second fiber connectors 734, 744, depending on the state ofthe waveplate 740. Accordingly, a user, by rotating the waveplate 740and altering the polarization of light can control the amount of lightcoupled between the first and second optical fiber connectors 734, 744and thereby adjust the level of attenuation.

Preferably, the optical elements such as the first and second lenses738, 748, the rotatable waveplate 740 and the MacNeille polarizer 742comprise micro-optics or are sufficiently small to provide for a compactmodule. The elements in the housing 732 may be laser welded or otherwisesecurely fastened to a base of the housing. The housing 732 may besealed and thermally insulated as well. In various preferredembodiments, these modules conform to Telcordia standards andspecifications.

A particularly preferred embodiment for a fiber solid-state regenerativeamplifier system (2000) is shown in FIG. 33 b. The mode-locked Yboscillator (2100) operates at near 50 MHz with a chirped pulse widthafter the fiber stretcher (2200) between 2-100 ps. The mode-lockingmeans is a saturable absorber mirror (2001). The gain is provided by aYb: doped fiber (2002). The other output coupler is a chirped fibergrating (2003) that also provides for dispersion compensation. Thecenter wavelength is between 1030-1040 nm with a bandwidth between 5-20nm. The pulse is compressible to 100-300 fs. It is pumped in core by aconventional laser diode (2005) through a polarization maintaining WDM(2004). Side pumping the cladding is also suitable. The pulse energy isnearly 1 nJ after amplification. The fiber amplifier (2300) is slightlynonlinear. The spectral broadening is negligible but is dependent on theinput power to the fiber amplifier. The Yb: fiber (2011) isapproximately 3 meters long. It is also polarization preserving fiber.The Yb: fiber amplifier gain shapes and frequency shifts slightly theoutput. It is pumped co propagating by a conventional single mode laserdiode (2009) through a polarization maintaining WDM (2010).Counterpropagating pumping and cladding pumping are also suitable. Theoutput from the fiber amplifier is through a bulk collimator (2012) anda bulk isolator (2013). More than one isolator may be necessary at thispoint. Alternatively, an AOM pulse selector can be added to the end ofthe amplifier for isolation. A Faraday rotator and polarizer can be usedat this point to separate the input of the regenerative amplifier (2400)from the output to the bulk grating compressor (2500). In addition thereis an isolator (2007) between the fiber stretcher and fiber amplifierthat includes an optical tap. The tap (2007) provides an optical syncoutput (2008) that is converted to an electrical signal by means of aphotodiode. This signal is used to synchronize the regenerativeamplifier pulse selector to the mode-locked fiber laser.

In this next embodiment an alternative source of the ultrafast pulses isa laser-diode or microchip laser. This embodiment is shown in FIGS. 34and 35. In FIG. 34, the microchip laser is a single longitudinalNd:vanadate source that provides a smooth temporal profile. The pulsewidth is 250 picoseconds. One solution for the compression fiber 62 is astandard single mode fiber with a mode field diameter of 5.9 μm and a NAof 0.12. The length of this compression fiber would be about 2 meters tocreate sufficient spectrum for a compression ratio of around 50. Theoutput energy from microchip lasers can be 10 microjoules. In this case,the light intensity at the entrance face of the fiber will be near thedamage threshold. A coreless end cap (not shown) can be used on thefiber so the mode can expand before the surface of the fiber. Otherwise,an amplifier with a larger mode field diameter can be used, such as amultimode fiber that propagates a single mode or a holey fiber amplifieras was used in (Furusawa et al “Cladding pumped Ytterbium-doped fiberlaser with holey inner and outer cladding”, Optics Express 9, pp.714-720, (2001)). If a fiber with an order of magnitude higher mode area(mode field diameter of 19.5 μm) is used, then the parameters in thefiber will be the same as in the case with 1 microjoule input. So thefiber length would again be 2 meters.

Since there is no interplay between dispersion and self-phase modulationin this design, the pulse width stays the same as the original pulsewidth. The nearly linear chirp is created by the shape of the pulse.Such a fiber is normally called a “compression fiber”. We propose toreplace this “compression fiber” with an amplifier fiber. The output ofthe amplifier will be a chirped pulse that can be compressed in acompressor. This saves the need of a stretcher.

For pulse energies significantly greater than 1 microjoule, the singlemode beam should be further amplified in a multimode fiber. This chirpedpulse source is ideal for amplification of ultrashort pulses by chirpedpulse amplification in a regenerative amplifier. The pulse is thencompressed after amplification. In this case the microchip 71 wasoperated at 0.5 μJ, and produced 250 ps, pulses and operating at therepetition rate of the regenerative amplifier. The compression fiber 62is now a multimode amplifier fiber that amplified a single mode with amode-field diameter of 17 μm. The pulse was then amplified to 30microjoules where Raman limited the amplification. This pulse is now achirped 250 ps pulse. It is further amplified in a solid stateregenerative amplifier and compressed in a bulk grating compressor totypically less than 1 ps. FIG. 35 illustrates the source generallydescribed in FIG. 3 of US Published Application 20040240037A1,incorporated by reference herein, with modification made to the chirpedfiber grating at the end of the source to further stretch the pulsesprior to amplification in the regenerative amplifier.

FIG. 36 illustrates a chirped pulse amplification system that utilizesconventional fiber stretchers, fiber amplifiers, bulk regenerativeamplifiers and bulk grating compressors. In order to obtain high qualitypulses from such systems, the control of higher-order dispersion andself-phase modulation is critical. A chirped pulse amplification systemallowing for independent control of second- and third order dispersionis shown in FIG. 36. In an exemplary embodiment, a seed source 101 basedon a passively modelocked Yb fiber laser was used. Such passivelymodelocked Yb fiber lasers were previously described in application Ser.No. 10/627,069 and are not further described here. The seed source 101produces positively chirped optical pulses with a bandwidth of 16nanometers at a repetition rate of 43 megahertz with an average power of16 milliwatts. The peak emission wavelength of the oscillator was 1053nanometers. The pulses from the seed source were compressible to a pulsewidth of less than 150 femtoseconds, demonstrating that the chirp fromthe seed source was approximately linear. The output from the seed laserpassed through an isolator (not shown) and a tunable bandpass filter 119with a 15 nanometer bandwidth.

After the bandpass filter 119, an output power of 5 milliwatts wasobtained and a fiber stretcher 120 was used to stretch the pulses to awidth of approximately 100 picoseconds. The fiber stretcher employed forproducing stretched pulses had a length of approximately 200 meters andwas based on conventional polarization maintaining single-modestep-index fiber. In FIG. 36, the tunable bandpass filter 119 is showninserted before the fiber stretcher 120; alternatively, the tunablebandpass filter 119 can also be inserted after the fiber stretcher 120(system implementation is not separately shown).

A subsequent Yb-based polarization maintaining pre-amplifier 121amplifies the stretched pulses to an average power of 500 milliwatts. Apulse picker 122, based on an acousto-optic modulator and pig-tailedwith polarization maintaining fiber, reduces the repetition rate of thepulses to 200 kilohertz, resulting in an average power of 1 milliwatt.The pulses from the pulse picker 122 were subsequently injected into alarge-mode polarization maintaining Yb fiber power amplifier 123 andamplified to an average power of 950 milliwatts. The Yb power amplifierhad a length of 3 meters and the fundamental mode spot size in the Ybpower amplifier was around 25 micrometers. All fibers were eitherspliced together with their polarization axes aligned or connected toeach other (with their polarization axes aligned) with appropriatemode-matching optics (not shown). The power amplifier 123 was claddingpumped via a lens 124 with a pump source 125, delivering a pump power ofabout 10 watts at a wavelength of 980 nanometers. A beam splittingmirror 126 was implemented to separate the pump light from the amplifiedsignal light. The amplified and stretched pulses from the poweramplifier 123 are further amplified in a bulk solid state regenerativeamplifier 129. The output pulses from the regenerative amplifier 129were compressed in a conventional bulk optics compressor 127 based on asingle diffraction grating with a groove density of 1200 lines/mm,operating near the Littrow angle. Such bulk optics compressors are wellknown in the state of the art and are not further explained here. Afterthe bulk optics compressor 127, the output 128 will contain pulses witha full-width half-maximum (FWHM) width of around 330 femtoseconds andpulse energies around 1 millijoule. Alternative designs should befeasible including a system without the power amplifier. However, inthis case the power amplifier is operating as the nonlinear fiberamplifier that is able to correct for higher order dispersion mismatchbetween the fiber stretcher and the bulk compressor.

Because stretched pulses can accumulate significant levels ofthird-order dispersion in the presence of self-phase modulation,gain-narrowing, gain-pulling and gain depletion, we refer to such pulsesas cubicons. More generally, we can define a cubicon as a pulse thatproduces controllable levels of at least linear and quadratic pulsechirp in the presence of at least substantial levels of self-phasemodulation (corresponding to a nonlinear phase delay>1) that can be atleast partially compensated by dispersive delay lines that producesignificant levels of second and third-order dispersion as well ashigher-order dispersion. (Please note that for the compensation oflinear pulse chirp, a dispersive delay line with second order dispersionis required, whereas for the compensation of quadratic pulse chirp, adispersive delay line with third order dispersion is required and so onfor higher orders of pulse chirp.) For a dispersive delay line toproduce a significant level of 2^(nd) and 3^(rd) as well as possiblyhigher-order dispersion, the stretched pulses are typically compressedby more than a factor of 30. In addition cubicons can also be formed inthe presence of resonant amplifier dispersion, gain narrowing, gainpulling as well as gain depletion, where we refer to gain depletion asan appreciable reduction in gain due to a single pulse. If a high powermode-locked oscillator an undoped fiber can be utilized to create theself-phase modulation. Spectral filtering will most likely be necessaryto obtain the appropriate pulse shape to the chirped pulse. The chirpedpulse width will need to be further expanded before amplification in theregenerative amplifier.

The importance of the pulse picker 122 has been described in Ser. No.10/960,923 in that it alleviates the specifications on the opticalswitch in the regenerative amplifier. A further advantage is that it canbe utilized as a variable attenuator for controlling the buildup time inthe regenerative amplifier. An AO switch can be used here, however EOswitches and EA switches are available in modules that conform toTelcordia standards and specifications. As pointed out in Ser. No.10/437,057 and 10/606,829, it often takes two switches since thestandard on off discrimination is 30 db while for lowering the rep ratefrom 30 MHz to 1 KHz requires an on off discrimination of more than 50db for the majority of the energy to be in the one pulse operating atthe lower repetition rate. Another use of the pulse picker is as avariable attenuator to control the nonlinearities in the fiber fordispersion correction. In cubicon amplification the nonlinearities arecritical for dispersion control and the variable attenuation feature ofthe pulse pickers is a means for controlling the nonlinear affects inthe fibers. Other variable attenuators can be used such as described inSer. No. 10/814,319. Other means of controlling the nonlinearities ofthe fiber amplifier are utilizing the control of the fiber amplifieroutput as described above. These include varying the gain or temperatureof the fiber amplifier by measuring the spectrum and or the outputintensity from the fiber amplifier. Controlling the spectrum and theintensity accurately for cubicon amplification can be implemented.

The embodiment of a short pulse source in the picosecond and nanosecondrange amplified in a fiber amplifier and amplified in a bulk amplifieris disclosed in application Ser. No. 10/927,374 (incorporated byreference herein) This system in some cases will have better performancewhen the bulk amplifier is utilized as a regenerative amplifier. Thisembodiment is shown in FIG. 38. Fiber amplifier system 501 is describedin detail in Ser. No. 10/927,374. The output pulse of the fiberamplifier system 501 is mode-matched by beam conditioning optics 506 tothe fundamental mode of the solid state regenerative amplifier 505. Theregenerative amplifier 505 utilizes a bulk crystal gain material whichis preferably directly diode pumped. The embodiment displayed in FIG. 38has the advantage that the gain bandwidth of the regenerative amplifiercan be matched to the fiber amplifier system. For example 1 ns pulseswith a spectral bandwidth of 0.6 nm and a pulse energy exceeding 100 μJ,centered at a wavelength of 1064 nm can be generated in a fiberamplifier chain in conjunction with a diode seed laser, for injectioninto a Nd:YVO₄ amplifier, which has a spectral bandwidth ofapproximately 0.9 nm. As another example a modelocked Yb-fiberoscillator with center wavelength of 1064 nm and a bandwidth of severalnm can be amplified and spectrally narrowed and matched to the gainbandwidth of the Nd:YVO₄ solid state amplifier. Thus 100 ps pulses withan energy of around 100 μJ and higher can be generated in a fiberamplifier chain and efficiently amplified in the regenerative amplifier.Without exploitation of spectral narrowing, the pulse energies fromfiber amplifier chains designed for the amplification of 100 ps pulsesin bulk Nd:YVO₄ amplifiers has to be reduced to avoid spectral clippingin the bulk amplifiers. Spectral narrowing is indeed universallyapplicable to provide high energy seed pulses for narrow line-widthsolid state amplifiers. For the example of bulk Nd:YVO₄ amplifiers,spectral narrowing is preferably implemented for pulse widths in therange of 20 ps-1000 ps.

Bulk solid-state regenerative amplifiers are also useful to increase theenergy of pulses generated with fiber based chirped pulse amplificationsystems. Chirped pulse amplification is generally employed to reducenonlinearities in optical amplifiers. The implementation of chirpedpulse amplification is most useful for the generation of pulses with awidth<50 ps. Due to the limited amount of pulse stretching andcompression that can be achieved with chirped pulse amplificationschemes, stretched pulses with an initial pulse width exceeding 1-5 nsare generally not implemented. Hence optical damage limits theachievable pulse energies from state of the art fiber based chirpedpulse amplification systems (assuming fiber power amplifiers with a corediameter of 30 μm) to around 1 mJ. Single stage bulk solid stateamplifiers can increase the achievable pulse energies normally by afactor of 10 while a regenerative amplifier has a gain of 10⁶. Thereforea regenerative amplifier can be preferable and give flexibility at acost of complexity. One advantage is significantly lower pulse energiescan be utilized from the fiber amplifier. A generic scheme 500 for theamplification of the output of a fiber based chirped pulse amplificationsystem in a bulk optical amplifier is shown in FIG. 39. Here short fs-pspulses with pulse energies of a few nJ are generated in fiber oscillator501. The pulses from the oscillator are stretched in pulse stretcher 502to a width of 100 ps-5 ns. The pulse stretcher is preferably constructedfrom a chirped fiber grating pulse stretcher as discussed with respectto FIG. 1 and can also be constructed from bulk optical gratings as wellknown in the state of the art. A pulse picker 503 reduces the repetitionrate of the oscillator to the 1 kHz-1 MHz range to increase the pulseenergy of the amplified pulses. A fiber amplifier chain represented by asingle fiber 504 is further used to increase the pulse energy to theμJ-mJ level. Appropriate mode matching optics 506 is then used to couplethe output of amplifier chain 504 into the bulk solid state amplifier505. Here bulk solid state amplifiers based on rods, slabs as well asthin disk concepts can be implemented. Appropriate bulk amplifiermaterial are based for example on Yb:YAG, Nd:YAG, Nd:YLF or Nd:YVO₄,Nd:glass, Yb, glass, Nd:KGW and others. Appropriate bulk amplifiermaterials and designs are well known in the state of the art and notfurther discussed here. A collimation lens 507 directs the output of thebulk solid state amplifier to the input of the compressor assembly. Tominimize the size of a chirped pulse amplification system employingnarrow bandwidth Nd-based crystals such as Nd:YAG, Nd:YLF, Nd:YVO₄ orNd:KGW the use of a grism based compressor is preferred. The opticalbeam is directed to via mirror 508 to the grism 509 and an additionalfolding prism 510 is used to minimize the size of the compressor. Mirror511 completes the compressor assembly. Such compressor assemblies havepreviously been used to compensate for third-order dispersion inwide-bandwidth chirped pulse amplification systems (i.e. chirped pulseamplification systems with a bandwidth>5 nm); no prior art existsapplying grism technology to narrow bandwidth chirped pulseamplification systems (i.e. chirped pulse amplification systemscomprising amplifiers with a spectral bandwidth<5 nm).

In an exemplary embodiment, fiber oscillator 501 generates 5 ps pulses,which are stretched by a chirped fiber grating stretcher to a width of 1ns. After amplification in the fiber amplifier chain a pulse energy of50 μJ is obtained at a repetition rate of 10 kHz. Further amplificationin a Nd:YVO₄ solid state booster amplifier generates a pulse energy of 2mJ. After recompression in the bulk grating compressor 10 ps pulses withan energy of 1 mJ are obtained. To ensure a compact design for the bulkgrating compressor, preferably grisms with a groove density of 2800 l/mmare implemented. The whole compressor can then fit into an area of about0.6×0.2 m by folding the optical beam path only once.

As discussed above, a burst of multiple pulses with differentwavelengths, different pulse widths and different temporal delays may bedesired. Referring to FIG. 40, an embodiment of the laser means 51 isillustrated, which increasing the increasing the possible energy andaverage power from ultrafast fiber lasers. A longer pulse envelope canbe obtained by utilizing a series of chirped gratings that reflect atdifferent wavelengths. After amplification, a similar series of gratingscan be placed to recombine/compress the pulses. In FIG. 40, pulses froma femtosecond pulse source are passed through an acousto-opticmodulator, a polarized beam-splitter and a Faraday rotator, and are thensupplied to a series of chirped fiber stretcher gratings that operate ondifferent portions of the input pulse spectrum. The spacings between thestretcher gratings can be l₁, l₂, l₃ . . . . In order to reconstruct thepulses after amplification in the fiber amplifier and the regenerativeamplifier the spacings between a series of complementary bulk glassBragg grating compressors are set to nl₁, nl₂, nl₃, . . . , where n isthe refractive index of the fiber between the stretcher fiber gratings,assuming that the bulk Bragg compression gratings are separated by air.The reconstructed pulse is output via a second beam splitter. Aspreviously mentioned, the reconstructed pulse is generally the result ofincoherent addition of the separately amplified spectral components ofthe input pulse.

If the distances between the compression and stretcher gratings are notequalized as described above, then multiple pulses will appear at theoutput. If the distances are not equal between the different sectionsthan the temporal delays will not be equal. This can be beneficial forapplications such as micro-machining. By varying the stretching andcompression ratios, pulses with different pulse widths can be generated.A single broadband compression grating can be used when generatingmultiple pulses.

The utilization of the regenerative amplifier is not as flexible as anall fiber amplifier system for modification of the pulse shape. Forexample, long pulse widths are limited to repetitive features equal tothe round trip time of the regenerative amplifier, e.g., approximately10 nanoseconds. For a regenerative amplifier, the pulse train created bythe gratings needs to be less than the round trip time of theregenerative amplifier.

Another embodiment of a multiple pulse source is shown in FIG. 41. Thissource is utilized in the laser system shown in FIG. 42. The Ytterbiumamplifier is normally needed for the pulse intensity to be sufficientfor amplification in the regenerative amplifier. The pulse compressor isoptional. The multiple pulse source is a laser diode and multipleelectronic drivers. In this case there are three sources with a delaygenerator that allows different delays to each electronic driver. A longpulse is generated by a conventional pulse driver for a laser diode. Theshorter pulses are derived from short pulse laser diode drivers such asare available from Avtech. These signals are added through electronicmixers. The output is shown in FIG. 43 a. This is an oscilloscope screenmeasured with a sufficiently fast photodiode. There are three peaksobservable. The output for one of the short pulses is shown in FIG. 43b. The pulse width is approximately 100 ps. FIG. 43 c illustrates athree peak pulse that is formed by changing the delay between the pulsesso the electronic signals overlap. The short pulses can also be chirpedand then recompressed to femtosecond pulses by the final compressor asdescribed in Ser. No. 08/312,912 and U.S. Pat. No. 5,400,350(incorporated by reference herein). By appropriately choosing the chirprates and frequency ranges a single bulk grating can compress aplurality of pulses.

Another embodiment of this is to utilize laser diodes at differentwavelengths or polarization states and then combine these opticallyeither with wavelength fiber combiners such as the wavelength routerutilized in multiple wavelength telecomm systems or by fiber splittersas shown in FIG. 44. It is also possible to utilize conventionalmode-locked sources to give multiple pulses. The methods for utilizingfiber gratings and etalons as disclosed in U.S. Pat. No. 5,627,848(incorporated by reference herein) as a source of multiple calibrationpulses can be utilized here. Another method is to use fiber splitterswith different path lengths as shown in FIG. 45. Four pulses are outputfor each pulse from the Ultrashort pulse source. The four pulses aresequentially, temporally delayed by:

-   -   1. c(2L_(N)+L₁+L₄)    -   2. c(2L_(N)+L₁+L₃)    -   3. c(2L_(N)+L₂+L₄)    -   4. c(2L_(N)+L₂+L₃)

1. A pulse source for generating pulses in the picosecond to nanosecondrange, comprising: a seed source emitting seed pulses, said sourcecomprising a Yb: mode-locked fiber oscillator, a laser diode, or amicrochip laser; at least one fiber amplifier optically coupled to saidseed source, and comprising a multimode fiber amplifier configured topropagate a single mode beam; a feedback controller coupled to one ormore of said seed source and said at least one fiber amplifier; a bulkoptical amplifier receiving amplified pulses from said at least onefiber amplifier and producing output pulses having pulse widths in thepicosecond to nanosecond range; and one or more pump laser diodes forpumping said seed source, said at least one fiber amplifier, and saidbulk optical amplifier.
 2. A pulse source, comprising: a seed sourcegenerating seed pulses, said seed source comprising a mode-locked fiberoscillator; a monitor for one or more parameters of said pulse source,said monitor operatively coupled to at least said mode locked fiberoscillator and responsive to one of more parameters of said mode lockedfiber oscillator; a pulse stretcher comprising a first length of opticalfiber, said pulse stretcher configured to receive and temporally stretchsaid seed pulses; a bulk optical amplifier configured to receive andamplify said temporally stretched seed pulses and to produce temporallystretched and amplified pulses; a pulse compressor disposed downstreamfrom said bulk optical amplifier, said pulse compressor configured tocompress said temporally stretched and amplified pulses and to producecompressed output pulses; and one or more pump laser diodes for pumpingsaid seed source and said bulk optical amplifier.
 3. The pulse sourceaccording to claim 2, wherein said one or more parameters comprise atleast a frequency of said mode locked fiber oscillator.
 4. The pulsesource according to claim 2, wherein at least one of said one or moreparameters is representative of mode lock startup of said mode lockedfiber oscillator.
 5. The pulse source according to claim 2, wherein atleast one of said one or more parameters is both measurable andcontrollable by said monitor.
 6. The pulse source according to claim 2,said pulse source comprising at least one fiber amplifier opticallycoupled to said seed source and disposed upstream from said bulk opticalamplifier, said at least one fiber amplifier increasing pulse energyprior to amplification with said bulk optical amplifier.
 7. The pulsesource according to claim 6, wherein said at least one fiber amplifierand said seed source are configured as a fiber-based MOPA.
 8. The pulsesource according to claim 2, wherein said mode-locked fiber oscillatorgenerates broadband pulses having a bandwidth comparable to thebandwidth of the mode-locked fiber oscillator gain medium; and whereinsaid seed source comprises a first pulse compressor and/or dispersioncompensator, said seed source generating seed pulses having pulse widthsin the range from about 20 fs to about 200 fs.
 9. The pulse sourceaccording to claim 2, wherein said monitor is operatively coupled to atleast one of said seed source, said bulk optical amplifier, or said oneor more pump diodes.
 10. The pulse source according to claim 2, whereinsaid monitor comprises a first monitor component, and a driver betweensaid mode locked fiber oscillator and said first monitor component. 11.The pulse source according to claim 6, wherein said monitor isoperatively coupled to said at least one fiber amplifier.
 12. The pulsesource according to claim 2, wherein said monitor comprises a sensor anda controller.
 13. The pulse source according to claim 2, wherein saidmonitor is configured to adjust an oscillator parameter.
 14. The pulsesource according to claim 6, wherein said monitor is configured toadjust a parameter of said at least one fiber amplifier.
 15. The pulsesource according to claim 2, wherein said monitor comprises a controlleroperatively coupled to said seed source, said monitor being configuredto adjust at least one of mode locked frequency, optical power, and modelock calibration.
 16. The pulse source according to claim 2, whereinsaid monitor comprises a controller having at least one module forextracting a tap signal for feedback control.
 17. The pulse sourceaccording to claim 2, wherein said monitor comprises a device formonitoring one or more of pulse width, wavelength, repetition rate,polarization and temporal delay characteristics of pulses produced bysaid seed source.
 18. The pulse source according to claim 2, whereinsaid monitor comprises a controller configured to control one or more ofa power and spectrum generated with said pulse source.
 19. The pulsesource according to claim 2, wherein said monitor comprises a controllerhaving a mode-lock start-up and search algorithm.
 20. The pulse sourceaccording to claim 2, wherein said pulse stretcher comprises a fiberBragg grating.
 21. The pulse source according to claim 20, wherein saidfiber Bragg grating comprises a chirped fiber Bragg grating.
 22. Thepulse source according to claim 2, wherein said pulse stretchercomprises at least one of optical fiber, dispersion shifted fiber, andchirped fiber Bragg grating.
 23. The pulse source according to claim 6,wherein said first length of optical fiber comprises a dispersionshifted and non-dispersion-shifted fiber.
 24. The pulse source accordingto claim 2, wherein said pulse stretcher comprises a second length ofoptical fiber and a fiber Bragg grating.
 25. The pulse source accordingto claim 2, wherein said first length of optical fiber comprises adispersion shifted and dispersion non-shifted fiber.
 26. The pulsesource according to claim 2, wherein a gain medium of said bulk opticalamplifier comprises Yb:YAG, Nd:YAG, Nd:YLF, Nd:YVO₄, Nd:glass, Yb,glass, Nd:KGW, or a narrow bandwidth Nd-based crystal.
 27. The pulsesource according to claim 2, wherein said bulk optical amplifiercomprises a regenerative amplifier.
 28. The pulse source according toclaim 2, wherein said bulk optical amplifier comprises a rod, slab, orthin disk.
 29. The pulse source according to claim 2, said pulse sourcecomprising an electro-optic or acousto-optic modulator disposed betweensaid seed source and said bulk optical amplifier, said modulatorconfigured as a pulse picker to reduce a repetition rate to within arange of 1 KHz to 1 MHz.
 30. The pulse source according to claim 2, saidpulse source further comprising a wavelength converter following anoutput of said bulk optical amplifier.
 31. The pulse source according toclaim 2, wherein said pulse compressor recompresses the pulses emittedfrom said bulk optical amplifier to near the bandwidth limit, saidcompressor comprising one or more of a bulk optic component, singlegrating compressor, a dispersive prism, a grism, a dispersive delayline, a fiber or bulk Bragg grating, and a chirped periodically poledcrystal.
 32. The pulse source according to claim 2, wherein said pulsecompressor is configured to produce femtosecond (fs) output pulses. 33.The pulse source according to claim 6, wherein said at least one fiberamplifier comprises a non-linear amplifier coupled to said seed source,said non-linear amplifier configured to generate spectrally narrowedamplified pulses matched to a gain bandwidth of said bulk opticalamplifier.
 34. The pulse source according to claim 14, wherein saidparameter comprises a gain of said at least one fiber amplifier.
 35. Thepulse source according to claim 2, wherein said monitor comprises a beammanipulator comprising: one or more of an optical gating device, a powermeter, a non-linear crystal, and a spectrometer.
 36. The pulse sourceaccording to claim 2, said pulse source comprising a pulse conditionerdisposed between said seed source and said bulk optical amplifier, saidpulse conditioner configured for at least one of: spectral narrowing,broadening or shaping, wavelength converting, temporal pulse compressionor stretching, pulse attenuation, or lowering of the repetition rate ofa pulse train generated by said seed source.
 37. The pulse sourceaccording to claim 2, wherein said passively modelocked oscillatorcomprises: concatenated sections of optical fiber comprising at leastone section of polarization maintaining fiber.
 38. A pulse source,comprising: a seed source generating seed pulses, said seed sourcecomprising a mode-locked fiber oscillator; a monitor for one or moreparameters of said pulse source, said monitor operatively coupled tosaid pulse source and comprising a sensor and a controller, said monitorreceiving a signal representative of said one or more parameters of saidpulse source; a pulse stretcher comprising a first length of opticalfiber, said pulse stretcher configured to receive and temporally stretchsaid seed pulses; a bulk optical amplifier configured to receive andamplify said temporally stretched seed pulses and to produce temporallystretched and amplified pulses; a pulse compressor disposed downstreamfrom said bulk optical amplifier, said pulse compressor configured tocompress said temporally stretched and amplified pulses and to producecompressed output pulses; and one or more pump laser diodes for pumpingsaid seed source and said bulk optical amplifier.
 39. The pulse sourceaccording to claim 37, said pulse source comprising at least one fiberamplifier optically coupled to said seed source and disposed upstreamfrom said bulk optical amplifier, said at least one fiber amplifierincreasing pulse energy prior to amplification with said bulk opticalamplifier.
 40. A pulse source, comprising: a seed source generating seedpulses, said seed source comprising a mode-locked fiber oscillator; amonitor for one or more performance parameters of said pulse source andcomprising a sensor arranged after said oscillator; a pulse stretchercomprising a first length of optical fiber, said pulse stretcherconfigured to receive and temporally stretch said seed pulses; a bulkoptical amplifier configured to receive and amplify said temporallystretched seed pulses and to produce temporally stretched and amplifiedpulses; a pulse compressor disposed downstream from said bulk opticalamplifier, said pulse compressor configured to compress said temporallystretched and amplified pulses and to produce compressed output pulses;and one or more pump laser diodes for pumping said seed source and saidbulk optical amplifier.
 41. A pulse source, comprising: a seed sourcegenerating seed pulses, said seed source comprising a mode-locked fiberoscillator; a monitor for one or more performance parameters of saidpulse source; a pulse stretcher comprising a first length of opticalfiber, said pulse stretcher configured to receive and temporally stretchsaid seed pulses; a bulk optical amplifier configured to receive andamplify said temporally stretched seed pulses and to produce temporallystretched and amplified pulses; a pulse compressor disposed downstreamfrom said bulk optical amplifier, said pulse compressor configured tocompress said temporally stretched and amplified pulses and to producecompressed output pulses; and one or more pump laser diodes for pumpingsaid seed source and said bulk optical amplifier. wherein said monitoris operatively coupled to at least one of said seed source, said bulkoptical amplifier, or said one or more pump diodes.
 42. The pulse sourceaccording to claim 6, wherein said pulse stretcher comprises a chirpedfiber Bragg grating.
 43. The pulse source according to claim 6, whereinsaid pulse stretcher comprises at least one of optical fiber, dispersionshifted fiber, and chirped fiber Bragg grating.
 44. The pulse sourceaccording to claim 6, wherein said pulse stretcher comprises a secondlength of optical fiber and a fiber Bragg grating.